Revert "cw1200: Don't perform SPI transfers in interrupt context"

This reverts commit aec8e88c.

This solution turned out to cause interrupt delivery problems, and
rather than trying to fix this approach, it has been scrapped in favor
of an alternative (and far simpler) implementation.

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