bnxt_en: Report firmware DCBX agent.

Report DCB_CAP_DCBX_LLD_MANAGED only if the firmware DCBX agent is enabled
and running for PF or VF.  Otherwise, if both LLDP and DCBX agents are
disabled in firmware, we report DCB_CAP_DCBX_LLD_HOST and allow host
IEEE DCB settings.  This patch refines the current logic in the driver.

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