Ensure TRX invariant
Ensure TRX invariant

There is implicit invariant in trx_phy_instance() which is actively used
by various hw-specific implementations to get TRX's phy instance. Let's
make sure there's explicit assertion for this because there's been
segfaults in the past related to it.

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