wl12xx: Change claiming of the SDIO bus

The SDIO bus is claimed and released for each SDIO transaction. In
addition to the few CPU cycles it takes to claim and release the bus, it
may also cause undesired side effects such as the MMC host stopping its
internal clocks.

Since only the wl12xx_sdio driver drives this SDIO card, it is safe to
claim the SDIO host once (on power on), and release it only when turning
the power off.

This patch was inspired by Juuso Oikarinen's (juuso.oikarinen@nokia.com)
patch "wl12xx: Change claiming of the (SDIO) bus".

Signed-off-by: Ido Yariv <ido@wizery.com>
Reviewed-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>