libertas: Firmware loading simplifications

Remove the ability to pass module parameters with firmware filenames
for USB and SDIO interfaces.

Remove the ability to pass custom "user" filenames to lbs_get_firmware().

Remove the ability to reprogram internal device memory with a different
firmware from the USB driver (we don't know of any users), and simplify
the OLPC firmware loading quirk to simply placing the OLPC firmware
at the top of the list (we don't know of any users other than OLPC).

Move lbs_get_firmware() into its own file.

These simplifications should have no real-life effect but make the
upcoming transition to asynchronous firmware loading considerably less
painful.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>