GPIO: OMAP: fix setting IRQWAKEN bits for OMAP4

Setting the IRQWAKEN bit was overwriting previous IRQWAKEN bits,
causing only the last bit set to take effect, resulting in lost
wakeups when the GPIO controller is in idle.

Replace direct writes to IRQWAKEN with MOD_REG_BIT calls to
perform a read-modify-write on the register.

Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>