bts-trx: early return on POWERON success
bts-trx: early return on POWERON success

Later on it's expected to add more rsp parsing, so let's return here and
not depend on generic return further down.

Change-Id: I8ddee46374ab329c47dc68e720e236f13d393f66

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