wl12xx_sdio: set MMC_PM_KEEP_POWER flag on suspend

if a wow trigger was configured, set the MMC_PM_KEEP_POWER flag
on suspend, so our power will be kept while the system is suspended.

We needed to set this flag on each suspend attempt (when we want
to keep power)

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>