ms: Add tbf() method to get the TBF based on the direction

To avoid the need for a switch or conditional statement when needing
a TBF from an MS object in direction independant code, this method
contains that case distinction. For additional flexibility, a
reverse() function is added to get the opposite direction.

Sponsored-by: On-Waves ehf
diff --git a/tests/ms/MsTest.cpp b/tests/ms/MsTest.cpp
index f2faefc..5a25d76 100644
--- a/tests/ms/MsTest.cpp
+++ b/tests/ms/MsTest.cpp
@@ -69,6 +69,9 @@
 	OSMO_ASSERT(ms->ul_tbf() == ul_tbf);
 	OSMO_ASSERT(ms->dl_tbf() == dl_tbf);
 
+	OSMO_ASSERT(ms->tbf(GPRS_RLCMAC_UL_TBF) == ul_tbf);
+	OSMO_ASSERT(ms->tbf(GPRS_RLCMAC_DL_TBF) == dl_tbf);
+
 	ms->detach_tbf(ul_tbf);
 	OSMO_ASSERT(!ms->is_idle());
 	OSMO_ASSERT(ms->ul_tbf() == NULL);