libertas: implement function init/shutdown commands for SD8688

SD8688 is a WLAN/Bluetooth combo chip and both functions are supported
in a single firmware image. FUNC_INIT and FUNC_SHUTDOWN commands are
implemented to utilize the multiple function feature.

When SD8688 card is inserted, the firmware image should be downloaded
only once through either WLAN function (Libertas driver) or Bluetooth
function (Bluetooth driver).

This patch adds function init/shutdown for SD8688 WLAN function only.

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