bna: Implement ethtool flash_device entry point.

Incorporated review comments from Ben Hutchings.

Change details:
	- Implement ethtool flash_device() entry point to write the
	  firmware image to the flash firmware partition.

Signed-off-by: Krishna Gudipati <kgudipat@brocade.com>
Reviewed-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>