iwlegacy: fix IBSS cleanup

We do not correctly change interface type when switching from
IBSS mode to STA mode, that results in microcode errors.

Resolves:
https://bugzilla.redhat.com/show_bug.cgi?id=886946

Reported-by: Jaroslav Skarvada <jskarvad@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>