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>