bnx2x: Allow ethtool to enable/disable loopback.

This patch updates the bnx2x_set_features() to handle loopback mode.
When enabled; it sets internal-MAC loopback.

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>