gprs_bssgp.c: Cast const of TLVP_VAL away.

Fix a compiler warning, we cast the const away at various
other parts in the code as well. We should consider removing
the const from the TLV struct..
diff --git a/openbsc/src/gprs/gprs_bssgp.c b/openbsc/src/gprs/gprs_bssgp.c
index d5ff44d..0d60ac9 100644
--- a/openbsc/src/gprs/gprs_bssgp.c
+++ b/openbsc/src/gprs/gprs_bssgp.c
@@ -256,8 +256,8 @@
 		return bssgp_tx_status(BSSGP_CAUSE_MISSING_MAND_IE, NULL, msg);
 
 	/* store pointer to LLC header and CELL ID in msgb->cb */
-	msgb_llch(msg) = TLVP_VAL(tp, BSSGP_IE_LLC_PDU);
-	msgb_bcid(msg) = TLVP_VAL(tp, BSSGP_IE_CELL_ID);
+	msgb_llch(msg) = (uint8_t *) TLVP_VAL(tp, BSSGP_IE_LLC_PDU);
+	msgb_bcid(msg) = (uint8_t *) TLVP_VAL(tp, BSSGP_IE_CELL_ID);
 
 	return gprs_llc_rcvmsg(msg, tp);
 }