rsl: Conditionally decrease log level if cause is normal event
rsl: Conditionally decrease log level if cause is normal event

This avoid a NOTICE log line like the following when gsm_lchan_release()
is called.
"""
rsl.c:2484 (bts=0,trx=0,ts=2,ss=0) Sending RTP delete indication: cause = Normal event, unspecified
"""
Change-Id: I0ce78c52644983220f5810bc5c661b07afd9e543

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