bnxt_en: refactor speed independent ethtool modes

A future patch in this series will change the algorithm used to
determine ethtool speed and media modes. Extract the handling of
the unrelated pause, autoneg modes into an independent function.
Also separate FEC handling out of bnxt_fw_to_ethtool_*_spds().

No functional change.

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