[connman] Check interface validity before setting regdom

Ofono may emit a signal with cellular network MCC value, resulting in
a call to wifi_set_regdom(), when the wifi_data structure has been
created but wifi->interface is still NULL. Check the pointer value
before proceeding.

The error condition is difficult to reproduce; mostly it seems to
happen when wlan tethering is active.
Merge pull request #82 from hmallat/wifi-regdom-param-check

[connman] Check interface validity before setting regdom