[rilmodem] Check if really network registration status is really searching

If connection drops to searching and only voice call registration
status changes to roaming, with current implementation it is possible
to get through roaming allowed check in core. This prevents it.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>
Merge pull request #201 from jkangas/next

Request for pulling checking of voice network status if data reg status is searching to next branch