wl1251: use wiphy_dev instead of wl->spi->dev

Remove a dependency on the bus-specific struct device by using wiphy_dev
when requesting firmware.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>