wl12xx: enter/exit psm on wowlan suspend/resume

When operating as station, enter psm before suspending
the device into wowlan state.

Add a new completion event to signal when psm was entered
successfully.

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