mwl8k: allow for different receive descriptor formats

As the receive descriptor format is determined by the firmware running
on the hardware and not by the hardware itself, and as these
descriptor formats vary a bit between different firmware releases,
abstract out the receive descriptor init/refill/process methods, and
allow choosing between different formats at run time depending on the
chip and firmware we're running on.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>