bnx2x: Correct PFC disablement

bnx2x driver could only have enabled pfc via usage of dcbnl; now, it can
also correctly disable it.

Signed-off-by: Barak Witkowski <barak@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>