rlc: Add missing break after GPRS_RLCMAC_CONTROL_BLOCK_OPT

Do not claim that the payload is not known. Add the missing break.

Do not print:
<0002> gprs_rlcmac.cpp:1174 GPRS_RLCMAC_CONTROL_BLOCK_OPT block payload is not supported.
<0002> gprs_rlcmac.cpp:1176 Unknown RLCMAC block payload.
diff --git a/src/gprs_rlcmac.cpp b/src/gprs_rlcmac.cpp
index 2fdc573..4f39bee 100644
--- a/src/gprs_rlcmac.cpp
+++ b/src/gprs_rlcmac.cpp
@@ -1172,6 +1172,7 @@
 		break;
 	case GPRS_RLCMAC_CONTROL_BLOCK_OPT:
 		LOGP(DRLCMAC, LOGL_NOTICE, "GPRS_RLCMAC_CONTROL_BLOCK_OPT block payload is not supported.\n");
+		break;
 	default:
 		LOGP(DRLCMAC, LOGL_NOTICE, "Unknown RLCMAC block payload.\n");
 		rc = -EINVAL;