[PATCH] libertas: split module into two (libertas.ko and usb8xxx.ko)

* add CONFIG_LIBERTAS to Kconfig
* remove global variable libertas_fw_name, the USB module might want to
    use a different default FW name than the CF module, so libertas_fw_name
    is now local to if_usb.c
* exported some symbols as GPL

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>