tbf: Make snd_ul_ud() and assemble_forward_llc() methods of UL TBF

They are only called for UL TBF.

Ticket: SYS#389
Sponsored by: On-Waves ehf
diff --git a/src/tbf.cpp b/src/tbf.cpp
index 22b8ce0..b5bb123 100644
--- a/src/tbf.cpp
+++ b/src/tbf.cpp
@@ -728,7 +728,7 @@
  * Store received block data in LLC message(s) and forward to SGSN
  * if complete.
  */
-int gprs_rlcmac_tbf::assemble_forward_llc(const gprs_rlc_data *_data)
+int gprs_rlcmac_ul_tbf::assemble_forward_llc(const gprs_rlc_data *_data)
 {
 	const uint8_t *data = _data->block;
 	uint8_t len = _data->len;
@@ -1749,7 +1749,7 @@
 }
 
 /* Send Uplink unit-data to SGSN. */
-int gprs_rlcmac_tbf::snd_ul_ud()
+int gprs_rlcmac_ul_tbf::snd_ul_ud()
 {
 	uint8_t qos_profile[3];
 	struct msgb *llc_pdu;