iuup_cn_node.c: make it work for AMR 12.2k codec

rx_data: prepend hardcoded AMR 12.2 header "0xf03c" to the front of RTP
payload
osmo_iuup_cn_tx_payload: strip AMR header (2 bytes) from the front of RTP
payload
diff --git a/tests/iuup/iuup_test.ok b/tests/iuup/iuup_test.ok
index 2b09c66..8c473d6 100644
--- a/tests/iuup/iuup_test.ok
+++ b/tests/iuup/iuup_test.ok
@@ -15,14 +15,14 @@
 8060944c6256042c000101020100e2b36cfb23bc46d18180c3e5ffe040045600005a7d35b625b80005fff03214ced0
 rx_payload() invoked by iuup_cn!
         [IuUP] -RTP->
-8060944c6256042c000101026cfb23bc46d18180c3e5ffe040045600005a7d35b625b80005fff03214ced0
+8060944c6256042c00010102f03c6cfb23bc46d18180c3e5ffe040045600005a7d35b625b80005fff03214ced0
 node_priv=0x2342
 ok: matches expected msg
 rc=0
 
 Transmit RTP. Expecting tx_msg() with inserted IuUP header
         [IuUP] <-RTP-
-8060944c6256042c000101026cfb23bc46d18180c3e5ffe040045600005a7d35b625b80005fff03214ced0
+8060944c6256042c00010102f03c6cfb23bc46d18180c3e5ffe040045600005a7d35b625b80005fff03214ced0
 tx_msg() invoked by iuup_cn!
  <-PDU- [IuUP]
 8060944c6256042c00010102000002b36cfb23bc46d18180c3e5ffe040045600005a7d35b625b80005fff03214ced0
@@ -32,7 +32,7 @@
 
 More RTP, each time the Frame Nr advances, causing a new header CRC.
         [IuUP] <-RTP-
-8060944c6256042c000101026cfb23bc46d18180c3e5ffe040045600005a7d35b625b80005fff03214ced0
+8060944c6256042c00010102f03c6cfb23bc46d18180c3e5ffe040045600005a7d35b625b80005fff03214ced0
 tx_msg() invoked by iuup_cn!
  <-PDU- [IuUP]
 8060944c6256042c000101020100e2b36cfb23bc46d18180c3e5ffe040045600005a7d35b625b80005fff03214ced0
@@ -40,7 +40,7 @@
 ok: matches expected msg
 rc=0
         [IuUP] <-RTP-
-8060944c6256042c000101026cfb23bc46d18180c3e5ffe040045600005a7d35b625b80005fff03214ced0
+8060944c6256042c00010102f03c6cfb23bc46d18180c3e5ffe040045600005a7d35b625b80005fff03214ced0
 tx_msg() invoked by iuup_cn!
  <-PDU- [IuUP]
 8060944c6256042c0001010202007eb36cfb23bc46d18180c3e5ffe040045600005a7d35b625b80005fff03214ced0
@@ -48,7 +48,7 @@
 ok: matches expected msg
 rc=0
         [IuUP] <-RTP-
-8060944c6256042c000101026cfb23bc46d18180c3e5ffe040045600005a7d35b625b80005fff03214ced0
+8060944c6256042c00010102f03c6cfb23bc46d18180c3e5ffe040045600005a7d35b625b80005fff03214ced0
 tx_msg() invoked by iuup_cn!
  <-PDU- [IuUP]
 8060944c6256042c0001010203009eb36cfb23bc46d18180c3e5ffe040045600005a7d35b625b80005fff03214ced0