mmc: sdhci: Properly enable SDIO IRQ wakeups

A little more work was needed for SDIO IRQ wakeups to be functional.

Wake-on-WLAN on the SD WiFi adapter in the XO-1.5 laptop is now working.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Chris Ball <cjb@laptop.org>