libertas: allow for PS mode to be disabled when firmware doesn't support it

Otherwise, we go into an endless busy loop trying to enable PS mode when
the command queue is empty, dealing with the error response, and then
trying to enable PS mode again because the command queue is empty.... it
doesn't really save much power.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>