libertas: EHS_REMOVE_WAKEUP is not always supported

Certain firmware versions, particularly the 8388 found on the XO-1,
do not support the EHS_REMOVE_WAKEUP command that is used to disable
WOL. Sending this command to the card will return a failure that
would get propagated up the stack and cause suspend to fail.

Instead, fall back to an all-zero wakeup mask.

This fixes http://dev.laptop.org/ticket/9967



Signed-off-by: Deepak Saxena <dsaxena@laptop.org>
Signed-off-by: Daniel Drake <dsd@laptop.org>
[includes fixups by Paul Fox]
Signed-off-by: John W. Linville <linville@tuxdriver.com>