rsl.c: Log RTP socket related errors as DRTP, not DRSL
rsl.c: Log RTP socket related errors as DRTP, not DRSL

The respective errors/events occur as a result of calling osmo_rtp_*
API, and are clearly more fitting into the DRTP category than the DRSL,
even though the respective actions are triggered by RSL.

Change-Id: I52e6f9865492a2f757a37860eb92a3dc49e174ef

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