wl1251: add support for dedicated IRQ line

wl1251 has WLAN_IRQ pin for generating interrupts to host processor,
which is mandatory in SPI mode and optional in SDIO mode (which can
use SDIO interrupts instead). However TI recommends using deditated
IRQ line for SDIO too.

Add support for using dedicated interrupt line with SDIO, but also leave
ability to switch to SDIO interrupts in case it's needed.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Reviewed-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>