vxge: add support for ethtool firmware flashing

Add the ability in the vxge driver to flash firmware via ethtool.

Updated to include comments from Ben Hutchings.

Signed-off-by: Jon Mason <jon.mason@exar.com>
Signed-off-by: Ram Vepa <ram.vepa@exar.com>
Signed-off-by: David S. Miller <davem@davemloft.net>