wireless: cw1200: Remove redundant spi driver bus initialization

In ancient times it was necessary to manually initialize the bus
field of an spi_driver to spi_bus_type. These days this is done in
spi_register_driver(), so we can drop the manual assignment.

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
To: Solomon Peachy <pizza@shaftnet.org>
To: Kalle Valo <kvalo@codeaurora.org>
To: linux-wireless@vger.kernel.org
To: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>