libertas: Added callback functions to support SDIO suspend/resume.

In suspend() host sleep is activated using already configured
host sleep parameters through wol command, and in resume() host
sleep is cancelled. Earlier priv->fw_ready flag used to reset and
set in suspend and resume handler respectively. Since after suspend
only host goes into sleep state and firmware is always ready, those
changes in flag state are removed.

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