gsm0808: add gsm0808_create_sapi_reject_cause()

This is basically a successor of gsm0808_create_sapi_reject(), but
instead of hard-coding GSM0808_CAUSE_BSS_NOT_EQUIPPED, it allows
the caller to specify a cause value to be used.  The old function
is now deprecated and should not be used.

Change-Id: Iefe5484d0fa02d5722b628b1dc237d51d3fb1a9b
Related: OS#4728
diff --git a/tests/gsm0808/gsm0808_test.c b/tests/gsm0808/gsm0808_test.c
index 5c1a931..dd2ffbe 100644
--- a/tests/gsm0808/gsm0808_test.c
+++ b/tests/gsm0808/gsm0808_test.c
@@ -374,7 +374,7 @@
 	struct msgb *msg;
 
 	printf("Testing creating SAPI Reject\n");
-	msg = gsm0808_create_sapi_reject(3);
+	msg = gsm0808_create_sapi_reject_cause(3, GSM0808_CAUSE_BSS_NOT_EQUIPPED);
 	VERIFY(msg, res, ARRAY_SIZE(res));
 	msgb_free(msg);
 }