bng_en: add support for ethtool -S stats display

bng_en: add support for ethtool -S stats display

Implement the legacy ethtool statistics interface (get_sset_count,
get_strings, get_ethtool_stats) to expose hardware counters not
available through standard kernel stats APIs.

Ex:
a) Per-queue ring stats
     rxq0_ucast_packets: 2
     rxq0_mcast_packets: 0
     rxq0_bcast_packets: 15
     rxq0_ucast_bytes: 120
     rxq0_mcast_bytes: 0
     rxq0_bcast_bytes: 900
     txq0_ucast_packets: 0
     txq0_mcast_packets: 0
     txq0_bcast_packets: 0
     txq0_ucast_bytes: 0
     txq0_mcast_bytes: 0
     txq0_bcast_bytes: 0

b) Per-queue TPA(LRO/GRO) stats
     rxq4_tpa_eligible_pkt: 0
     rxq4_tpa_eligible_bytes: 0
     rxq4_tpa_pkt: 0
     rxq4_tpa_bytes: 0
     rxq4_tpa_errors: 0
     rxq4_tpa_events: 0

c) Port level stats
     rxp_good_vlan_frames: 0
     rxp_mtu_err_frames: 0
     rxp_tagged_frames: 0
     rxp_double_tagged_frames: 0
     rxp_pfc_ena_frames_pri0: 0
     rxp_pfc_ena_frames_pri1: 0
     rxp_pfc_ena_frames_pri2: 0
     rxp_pfc_ena_frames_pri3: 0
     rxp_pfc_ena_frames_pri4: 0
     rxp_pfc_ena_frames_pri5: 0
     rxp_pfc_ena_frames_pri6: 0
     rxp_pfc_ena_frames_pri7: 0
     rxp_eee_lpi_events: 0
     rxp_eee_lpi_duration: 0
     rxp_runt_bytes: 0
     rxp_runt_frames: 0
     txp_good_vlan_frames: 0
     txp_jabber_frames: 0
     txp_fcs_err_frames: 0
     txp_pfc_ena_frames_pri0: 0
     txp_pfc_ena_frames_pri1: 0
     txp_pfc_ena_frames_pri2: 0
     txp_pfc_ena_frames_pri3: 0
     txp_pfc_ena_frames_pri4: 0
     txp_pfc_ena_frames_pri5: 0
     txp_pfc_ena_frames_pri6: 0
     txp_pfc_ena_frames_pri7: 0
     txp_eee_lpi_events: 0
     txp_eee_lpi_duration: 0
     txp_xthol_frames: 0

d) Per-priority stats
     rx_bytes_pri0: 4182650
     rx_bytes_pri1: 4182650
     rx_bytes_pri2: 4182650
     rx_bytes_pri3: 4182650
     rx_bytes_pri4: 4182650
     rx_bytes_pri5: 4182650
     rx_bytes_pri6: 4182650
     rx_bytes_pri7: 4182650

Signed-off-by: Bhargava Marreddy <bhargava.marreddy@broadcom.com>
Reviewed-by: Vikas Gupta <vikas.gupta@broadcom.com>
Link: https://patch.msgid.link/20260406180420.279470-11-bhargava.marreddy@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>