libertas: remove ps_supported flag, use fwcapinfo

Power save support depends on the firmware capabilities rather than the
card's hardware interface.  Use the FW_CAPINFO_PS bit in the firmware
capabilities mask throughout the driver in place of the redundant
ps_supported flag and don't make decisions about PS support in the
interface drivers (with the exception of a special case in the USB
driver).

V2: put the USB special case in the right place.

Signed-off-by: Andrey Yurovsky <andrey@cozybit.com>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>