osmo-bts-trx: make use of OSMO_UNLIKELY() when handling TRXD PDUs
osmo-bts-trx: make use of OSMO_UNLIKELY() when handling TRXD PDUs

Given how often we send and receive TRXD PDUs, I believe it maks
sense to use OSMO_UNLIKELY() in the error checking conditions.
Theoretically this should improve performance.

Change-Id: I9266aaf2bbc0acfe21e9bfb12c9f38621d120553

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts