bnx2x: display the correct duplex value

Prior to this fix, the driver reported the chip's active duplex state
is always 'full', even if using half-duplex mode.

Signed-off-by: Yaniv Rosner <yaniv.rosner@broadcom.com>
Signed-off-by: Yuval Mintz <yuvalmin@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>