[rilmodem] Hide searching state only when registered has been reported

Hiding searching status always might cause very rare case where very
brief voice call connection status change to something else than
registered might cause ofono core to force data connection down.
Since connection down is not immediate but status can change right
away back that causes situation where response for outdated deattach
request overrides the return to attached state.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>