libertas: [cs] use common firmware request helper and new firmware locations
    
    linux-firmware puts libertas firmware in /libertas.  Fix the driver to
    look there first, but fall back to the old firmware names if the new
    ones don't exist.
    
    Signed-off-by: Dan Williams <dcbw@redhat.com>
    Signed-off-by: John W. Linville <linville@tuxdriver.com>