Simplify tbf::set_polling()

When setting a POLL, it will always happen on PACCH, so all the CCCH
part makes no sense there. Let's drop it and move the logging of each
case to the caller, where logging file+line is more useful.

Change-Id: I242f97fd6f927131ac64c1a7c9c3812b6389de04
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index 905bf05..65785f2 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -958,6 +958,9 @@
 		rc = check_polling(fn, ts, &new_poll_fn, &rrbp);
 		if (rc >= 0) {
 			set_polling(new_poll_fn, ts, PDCH_ULC_POLL_DL_ACK);
+			LOGPTBFDL(this, LOGL_DEBUG,
+				  "Scheduled DL Acknowledgement polling on PACCH (FN=%d, TS=%d)\n",
+				  new_poll_fn, ts);
 
 			m_tx_counter = 0;
 			/* start timer whenever we send the final block */