bnxt_en: Implement ethtool set_fec_param() method.

This feature allows the user to set the different FEC modes on the NIC
port.  Any new setting will take effect immediately after a link toggle.

Reviewed-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>