bnxt_en: Disallow forced speed for 10GBaseT devices.

10GBaseT devices must autonegotiate to determine master/slave clocking.
Disallow forced speed in ethtool .set_settings() for these devices.

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>