gsm0808: Make a function to extract Cause IE publicly available.

Function gsm0808_get_cipher_reject_cause() was previously available
in private gsm0808_utils.h. In practice, the exact same code is useful
to extract Cause IE value from any of the many other BSSMAP messages
which use it.

So let's rename it to gsm0808_get_cause() and make it avilable
to everyone to use.

Change-Id: Idf2b99e9ef014eba26e3d4f0f38c2714d3a0520a
diff --git a/tests/gsm0808/gsm0808_test.c b/tests/gsm0808/gsm0808_test.c
index ec24914..ce73390 100644
--- a/tests/gsm0808/gsm0808_test.c
+++ b/tests/gsm0808/gsm0808_test.c
@@ -306,7 +306,7 @@
 	if (rc < 0)
 		printf("FIXME: failed (%d) to parse created message %s\n", rc, msgb_hexdump(msg));
 
-	rc = gsm0808_get_cipher_reject_cause(&tp);
+	rc = gsm0808_get_cause(&tp);
 	if (rc < 0)
 		printf("FIXME: failed (%s) to extract Cause from created message %s\n",
 		       strerror(-rc), msgb_hexdump(msg));