bts-trx: sched_lchan_tchh.c: Workaround gcc false positive error
bts-trx: sched_lchan_tchh.c: Workaround gcc false positive error

Manual analysis of code didn't end up in finding any issue, so this
seems a false positive (I can really understand gcc failing to do proper
job here, this function has way to many jumps here and there.

"""
/sched_lchan_tchh.c:88:13: error: ‘rc’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
   88 |         int rc, amr = 0;
      |             ^~
"""

Change-Id: Ifebaee63a9dad04976ffb4438c32360687ef095a

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