bnx2x: prevent timeouts when using PFC

Prevent updating the xmac PFC configuration when using a link speed
slower than 10G -the umac block is responsible for 1G or slower connections,
therefore it is possible the xmac block is reset when connection is slower.

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>