encoding: Change function signature to use UL TBFs where it makes sense

encode_rbb() and write_packet_uplink_assignment are only called with UL
TBFs so change the function signature to reflect that.

Ticket: SYS#389
Sponsored by: On-Waves ehf
diff --git a/src/encoding.h b/src/encoding.h
index e62c2c8..0004c42 100644
--- a/src/encoding.h
+++ b/src/encoding.h
@@ -47,7 +47,7 @@
 			struct gprs_rlcmac_bts *bts,
 			bitvec * dest, uint8_t old_tfi,
 			uint8_t old_downlink, uint32_t tlli, uint8_t use_tlli, 
-			struct gprs_rlcmac_tbf *tbf, uint8_t poll, uint8_t alpha,
+			struct gprs_rlcmac_ul_tbf *tbf, uint8_t poll, uint8_t alpha,
 			uint8_t gamma, int8_t ta_idx);
 
 	static void write_packet_downlink_assignment(RlcMacDownlink_t * block, uint8_t old_tfi,
@@ -56,7 +56,7 @@
 
 	static void encode_rbb(const char *show_rbb, uint8_t *rbb);
 
-	static void write_packet_uplink_ack(struct gprs_rlcmac_bts *bts, RlcMacDownlink_t * block, struct gprs_rlcmac_tbf *tbf,
+	static void write_packet_uplink_ack(struct gprs_rlcmac_bts *bts, RlcMacDownlink_t * block, struct gprs_rlcmac_ul_tbf *tbf,
 		        uint8_t final);
 
 	static int write_paging_request(bitvec * dest, uint8_t *ptmsi, uint16_t ptmsi_len);