Libertas: fix issues while configuring host sleep using ethtool wol

Configuration of wake-on-lan for unicast, multicast, broadcast, physical
activity was not working. Kernel panic issue was there when user tries to
disable WOL. Fixed them.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>