bnx2x: Fix 20G KR2 support claims

Don't claim 20G is supported if the speed is unsupported by the phys
(reflected by various ethtools and ndos).

Signed-off-by: Yaniv Rosner <yanivr@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>