osmo-bts-trx: fix a memleak in trx_sched_set_lchan()
osmo-bts-trx: fix a memleak in trx_sched_set_lchan()

An additional burst buffer is allocated in rx_data_fn() for Uplink
SACCH repetition, but unlike the main buffer it never gets free()d.

Change-Id: I93310e7aed91a49c0511e2ed30e000440617f95a
Fixes: I7e4cc33cc010866e41e3b594351a7f7bf93e08ac
Related: OS#4795, SYS#5114

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