bnxt_en: Display function level rx/tx_discard_pkts via ethtool

Add counters to display sum of rx/tx_discard_pkts of all rings as
function level statistics via ethtool.

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