ath5k: Adjust opmode when interfaces are removed.

Otherwise, if there is an AP and a STATION, and AP
is removed, the NIC will not revert back to STATION mode.

Reported-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>