libertas: [spi] 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.  Add preference for newer firmware versions too.

Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>