cw1200: Use a threaded oneshot irq handler for cw1200_spi

This supercedes the older patch ("cw1200: Don't perform SPI transfers in
interrupt context") that badly attempted to fix this problem.

This is a far simpler solution, which has the added benefit of
actually working.

Signed-off-by: Solomon Peachy <pizza@shaftnet.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>