bnx2x: Fix 848xx duplex settings

On 848xx PHY (10G-baseT), half-duplex was always advertised regardless of the
actual configuration. Change the 848xx duplex settings to advertise half-duplex
only if configured.

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>