libertas: add two-stage firmware request helper
    
    SDIO, GSPI, and CS all use 2-stage firmware and the loading
    process and logic should be the same.  Allow module parameters
    to override the automatic firmware choice, otherwise just walk
    the bus driver's firmware table and pick out the first firmware
    pair that exists for the given model.
    
    Some special care is taken to allow overriding of just the helper
    or the main firmware, but let the other of the pair be chosen
    automatically.
    
    Signed-off-by: Dan Williams <dcbw@redhat.com>
    Signed-off-by: John W. Linville <linville@tuxdriver.com>