libertas_usb: program OLPC EC wakeup mask for wake-on-WLAN

OLPC power management code has recently gone upstream. This piece
completes the puzzle for libertas_usb, which now programs the OLPC EC
for wlan wakeups when they have been requested.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>