bnxt_en: Properly report no pause support on some cards

Some cards are configured to never support link pause or PFC.  Discover
these cards and properly report no pause support to ethtool.  Disable
PFC settings from DCBNL if PFC is unsupported.

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