bnxt_en: Check status of firmware DCBX agent before setting DCB_CAP_DCBX_HOST.

Otherwise, all the host based DCBX settings from lldpad will fail if the
firmware DCBX agent is running.

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