mgcp: Fix/test reading/writing the sequence number

The sequence number was read from the wrong place and then
the wrong byte order conversion routine was used so we ended
up wirting 0x00, 0x00 into the patched sequence number. Add
a testcase for that.
diff --git a/openbsc/tests/mgcp/mgcp_transcoding_test.ok b/openbsc/tests/mgcp/mgcp_transcoding_test.ok
index 07929a8..e06b0e1 100644
--- a/openbsc/tests/mgcp/mgcp_transcoding_test.ok
+++ b/openbsc/tests/mgcp/mgcp_transcoding_test.ok
@@ -27,14 +27,14 @@
 == Transcoding test ==
 converting l16 -> gsm
 encoded:
-    80 0b 00 00 00 00 00 a0 11 22 33 44 d4 7c e3 e9 
+    80 0b 00 01 00 00 00 a0 11 22 33 44 d4 7c e3 e9 
     62 50 39 f0 f8 b4 68 ea 6c 0e 81 1b 56 2a d5 bc 
     69 9c d1 f0 66 7a ec 49 7a 33 3d 0a de 
 counted: 12
 == Transcoding test ==
 converting l16 -> pcma
 encoded:
-    80 0b 00 00 00 00 00 a0 11 22 33 44 d5 a5 a3 a5 
+    80 0b 00 01 00 00 00 a0 11 22 33 44 d5 a5 a3 a5 
     d5 25 23 25 d5 a5 a3 a5 d5 25 23 25 d5 a5 a3 a5 
     d5 25 23 25 d5 a5 a3 a5 d5 25 23 25 d5 a5 a3 a5 
     d5 25 23 25 d5 a5 a3 a5 d5 25 23 25 d5 a5 a3 a5 
@@ -49,7 +49,7 @@
 == Transcoding test ==
 converting gsm -> l16
 encoded:
-    80 03 00 00 00 00 00 a0 11 22 33 44 00 00 54 00 
+    80 03 00 01 00 00 00 a0 11 22 33 44 00 00 54 00 
     59 f0 34 20 c4 c8 b9 f8 e2 18 f1 e8 f2 28 f0 e0 
     46 08 4f 80 2c a0 a9 c8 80 00 c0 58 3f 80 63 c0 
     24 b8 fa b8 f6 88 0b a0 c8 70 a8 b0 c8 c0 3b a8 
@@ -81,7 +81,7 @@
 == Transcoding test ==
 converting gsm -> pcma
 encoded:
-    80 03 00 00 00 00 00 a0 11 22 33 44 d5 a0 a3 bf 
+    80 03 00 01 00 00 00 a0 11 22 33 44 d5 a0 a3 bf 
     38 24 08 19 1e 1b a4 a6 b3 20 2a 3a ba ad b7 60 
     17 92 3e 20 3e b8 ac b2 32 2c 20 02 b6 be be 82 
     04 27 26 35 8d a4 a6 b5 35 21 20 31 8d a7 a6 b6 
@@ -96,7 +96,7 @@
 == Transcoding test ==
 converting pcma -> l16
 encoded:
-    80 08 00 00 00 00 00 a0 11 22 33 44 00 08 42 00 
+    80 08 00 01 00 00 00 a0 11 22 33 44 00 08 42 00 
     5a 00 42 00 00 08 be 00 a6 00 be 00 00 08 42 00 
     5a 00 42 00 00 08 be 00 a6 00 be 00 00 08 42 00 
     5a 00 42 00 00 08 be 00 a6 00 be 00 00 08 42 00 
@@ -121,7 +121,7 @@
 == Transcoding test ==
 converting pcma -> gsm
 encoded:
-    80 08 00 00 00 00 00 a0 11 22 33 44 d4 b9 f4 5d 
+    80 08 00 01 00 00 00 a0 11 22 33 44 d4 b9 f4 5d 
     d9 50 5a e1 a0 cd 76 ea 52 0e 87 53 ad d4 ea a2 
     0a 63 ca e9 60 79 e2 2a 25 d2 c0 f3 39 
 counted: 12
@@ -161,7 +161,7 @@
 == Transcoding test ==
 converting gsm -> pcma
 encoded:
-    80 08 00 00 00 00 00 a0 11 22 33 44 d5 a0 a3 bf 
+    80 08 00 01 00 00 00 a0 11 22 33 44 d5 a0 a3 bf 
     38 24 08 19 1e 1b a4 a6 b3 20 2a 3a ba ad b7 60 
     17 92 3e 20 3e b8 ac b2 32 2c 20 02 b6 be be 82 
     04 27 26 35 8d a4 a6 b5 35 21 20 31 8d a7 a6 b6