pdch: Use helper function tbf_as_ul_tbf()

Change-Id: Ie2a0af7928578823e944b85781ce284722547661
diff --git a/src/pdch.cpp b/src/pdch.cpp
index 4d6d8f6..369406f 100644
--- a/src/pdch.cpp
+++ b/src/pdch.cpp
@@ -701,7 +701,7 @@
 				/* let common path expire the poll */
 				goto return_unref;
 			}
-			ul_tbf = (struct gprs_rlcmac_ul_tbf *)item->tbf_poll.poll_tbf;
+			ul_tbf = tbf_as_ul_tbf(item->tbf_poll.poll_tbf);
 			if (item->tbf_poll.reason != PDCH_ULC_POLL_UL_ACK) {
 				LOGPDCH(this, DRLCMAC, LOGL_NOTICE, "FN=%u PKT RESOURCE REQ: "
 					"Unexpectedly received, waiting for poll reason %d\n",