bnxt_en: Implement DCBNL to support host-based DCBX.

Support only IEEE DCBX initially.  Add IEEE DCBNL ops and functions to
get and set the hardware DCBX parameters.  The DCB code is conditional on
Kconfig CONFIG_BNXT_DCB.

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