libertas: Added support for host sleep feature

Existing "ethtool -s ethX wol X" command configures hostsleep
parameters, but those are activated only during suspend/resume,
there is no way to configure host sleep without actual suspend.

This patch adds debugfs command to enable/disable host sleep based on
already configured host sleep parameters using wol command.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kiran Divekar <dkiran@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>