bnx2x: correct reading of speed capabilities

When the bnx2x driver reads the port configuration - mask irrelevant bits.

Without this change, the unintended bits may cause the driver to needlessly
toggle the link, as a comparison in the link flap avoidance flow will show
that the old link did not advertise the same capabilities and thus cannot
be retained.

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>