bnx2x: Fix BCM578x0-SFI pre-emphasis settings

Fix 578x0-SFI pre-emphasis settings per HW recommendations to achieve better
link strength.

Signed-off-by: Yaniv Rosner <yanivr@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>