drivers: modem: wnc14a2a: dont stop RSSI processing on error

Instead of aborting on RSSI poll error, let's continue on and
reschedule the next RSSI poll.

This fixes an issue where RSSI polling would stop if for any reason
the modem doesn't fulfill an RSSI poll on time.

Signed-off-by: Michael Scott <mike@foundries.io>