wct4xxp: Report rx signalling bit changes after spanconfig.

This fixes a long standing issue where, for CAS signaling, the RX bits were
sometimes misreported after span configuration before the first detected state
change.

The logic in the wct4xxp driver now matches that in the wcte43x driver and
wcte13xp drivers. The wcte12xp driver always polls the sigbits due to how
voicebus works and is not affected by this.

Internal-Issue-ID: DAHDI-1081
Signed-off-by: Shaun Ruffell <sruffell@digium.com>