bnxt_en: Fix active FEC reporting to ethtool

ethtool --show-fec <interface> does not show anything when the Active
FEC setting in the chip is set to None.  Fix it to properly return
ETHTOOL_FEC_OFF in that case.

Fixes: 8b277589

 ("bnxt_en: Report FEC settings to ethtool.")
Signed-off-by: Somnath Kotur <somnath.kotur@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>