libertas: fix ps-mode related removal problems

When the device is remove e.g. because of going to suspend
 mode with powersaving enabled, lbs_remove_card tries to exit
powersaving state even when already woken up. That command is
not processed properly in that situation, since the command
processing queue is already stopped, so it waits forever
for the command being processed, so disable it.

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