wireless: cw1200: acquire hwbus lock around cw1200_irq_handler() call.

This fixes "lost interrupt" problems that occurred on SPI-based systems.
cw1200_irq_handler() expects the hwbus to be locked, but on the
SPI-path, that lock wasn't taken (unlike in the SDIO-path, where the
generic SDIO-code takes care of acquiring the lock).

Cc: stable@vger.kernel.org
Signed-off-by: David Mosberger <davidm@egauge.net>
Signed-off-by: Solomon Peachy <pizza@shaftnet.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>