bnx2x: add `ethtool -w' support.

This revises and enhances the bnx2x register dump facilities,
adding support for `ethtool -w' on top of `ethtool -d'.

Signed-off-by: Miriam Shitrit <miris@broadcom.com>
Signed-off-by: Yuval Mintz <yuvalmin@broadcom.com>
Signed-off-by: Ariel Elior <ariele@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>