Rename as_{dl,ul}_tbf -> tbf_as_{dl,ul}_tbf

Prefix the function accordingly to specify the input object.

Change-Id: I225568fe2cb5236817cdd9609354c24c66723606
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index 9fe1bd8..a8fb6fc 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -1318,7 +1318,7 @@
 	}
 }
 
-struct gprs_rlcmac_dl_tbf *as_dl_tbf(struct gprs_rlcmac_tbf *tbf)
+struct gprs_rlcmac_dl_tbf *tbf_as_dl_tbf(struct gprs_rlcmac_tbf *tbf)
 {
 	if (tbf && tbf->direction == GPRS_RLCMAC_DL_TBF)
 		return static_cast<gprs_rlcmac_dl_tbf *>(tbf);