Avoid using N3103 in DL TBFs

N3103 counts retransmits of PACKET UPLINK ACK/NACK with FinalACK=1. As a
consequence, this counter only applies to UL TBFs.

Current code is only using it in UL TBF, but add an assert to clarify
and make sure it is not used unproperly in the future.

Change-Id: I026d6145249ef19694f673ec7b4928af9d401dd6
---
M src/tbf.cpp
1 file changed, 19 insertions(+), 0 deletions(-)

diff --git a/src/tbf.cpp b/src/tbf.cpp
index bc5411a..47256f8 100644
--- a/src/tbf.cpp
+++ b/src/tbf.cpp
@@ -313,6 +313,9 @@
 	case N3101:
 		OSMO_ASSERT(direction == GPRS_RLCMAC_UL_TBF);
 		break;
+	case N3103:
+		OSMO_ASSERT(direction == GPRS_RLCMAC_UL_TBF);
+		break;
 	default:
 		break;
 	}
@@ -335,6 +338,7 @@
 		chk = bts->n3101;
 		break;
 	case N3103:
+		OSMO_ASSERT(direction == GPRS_RLCMAC_UL_TBF);
 		chk = bts->n3103;
 		break;
 	case N3105:

