wl12xx: Add support for block reading from a fixed register address

Add support for block reading (multiple bytes) from a fixed chipset
register address. This is required for the wl1271 TX data path.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>