nm_bts_fsm: Reset si_valid bitmask when BTS is shut down
nm_bts_fsm: Reset si_valid bitmask when BTS is shut down

Otherwise after reconnecting some SI are still marked as valid despite
the new BSC conn didn't set them.

Change-Id: I71e2a5cdf4e2e287db7b05df1a165a1e32c958ca

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