libertas: nuke useless variable usbdriver_name and useless comments

I think it was pretty obvious what fields in if_usb_driver are...

Signed-off-by: Andres Salomon <dilinger@debian.org>
Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>