bnxt_en: convert to linkmode_set_bit() API

Barring the BNXT_FW_TO_ETHTOOL speed macros, which will be removed
in the next patch, update code to use the newer API.

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>