Use GSM48_MT_RR_IMM_ASS instead of hardcoded 0x3f value

Change-Id: Id966cbab403763d006bd5e59757c5bf57402bf9d
diff --git a/src/pcu_l1_if.cpp b/src/pcu_l1_if.cpp
index 67e4de5..f964b78 100644
--- a/src/pcu_l1_if.cpp
+++ b/src/pcu_l1_if.cpp
@@ -510,7 +510,7 @@
 
 	switch (data_cnf->sapi) {
 	case PCU_IF_SAPI_PCH:
-		if (data_cnf->data[2] == 0x3f)
+		if (data_cnf->data[2] == GSM48_MT_RR_IMM_ASS)
 			bts_rcv_imm_ass_cnf(bts, data_cnf->data, data_cnf->fn);
 		break;
 	default: