bts, tbf: Change the TBF return type of functions to the ul/dl version

Many functions only ever deal with or return a UL or a DL TBF.
Explicitly change the type to reflect which TBF is used where.

Ticket: SYS#389
Sponsored-by: On-Waves ehf
diff --git a/src/tbf.h b/src/tbf.h
index 8464e19..d650021 100644
--- a/src/tbf.h
+++ b/src/tbf.h
@@ -259,7 +259,7 @@
 };
 
 
-struct gprs_rlcmac_tbf *tbf_alloc_ul(struct gprs_rlcmac_bts *bts,
+struct gprs_rlcmac_ul_tbf *tbf_alloc_ul(struct gprs_rlcmac_bts *bts,
 	int8_t use_trx, uint8_t ms_class,
 	uint32_t tlli, uint8_t ta, struct gprs_rlcmac_tbf *dl_tbf);