libertas: do not confirm sleep if commands are pending

If the main thread gets one PS AWAKE event and one PS SLEEP event
in one iteration over event_fifo there will never be checks for
commands to be processed, since psstate will always be
PS_STATE_SLEEP or PS_STATE_PRE_SLEEP

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>