Fix regression in 'osmo-bts-trx: rework and split up bts_sched_fn()'
Fix regression in 'osmo-bts-trx: rework and split up bts_sched_fn()'

In [1] I forgot to update tx_fcch_fn(), where we assume that the
burst buffer is already zero-initialized and do not call memset().
This results in sending dummy bursts on FCCH (instead of zeros).

Change-Id: I491b3b5702b3cfca45c12c002b41e60c629b05b9
Fixes: [1] Iec78989517865b3275a9784d1042a5ebd1d2815f

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