osmo-bts-trx: ignore frame offset error on startup
osmo-bts-trx: ignore frame offset error on startup

Previously we always printed error on startup:
l1sap.c:461 Invalid condition detected: Frame difference is 627202-0=627202 > 1!

This is expected and of little practical use. Let's suppress it
by checking that FN was actually initialized.

Change-Id: I4f7cf285e437db0e980e1a2a3ab48a7255945448

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