zd1211rw: cancel process_intr work on zd_chip_disable_int()

OOPS if worker is running and disconnect() is called (triggered
by unpluging device). Much harder to trigger at this stage but
later when we have AP beacon work in process_intr it happens very
easy.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>