bng_en: implement netdev_stat_ops

bng_en: implement netdev_stat_ops

Implement netdev_stat_ops to provide standardized per-queue
statistics via the Netlink API.

Below is the description of the hardware drop counters:

rx-hw-drop-overruns: Packets dropped by HW due to resource limitations
(e.g., no BDs available in the host ring).
rx-hw-drops: Total packets dropped by HW (sum of overruns and error
drops).
tx-hw-drop-errors: Packets dropped by HW because they were invalid or
malformed.
tx-hw-drops: Total packets dropped by HW (sum of resource limitations
and error drops).

The implementation was verified using the ynl tool:

./tools/net/ynl/pyynl/cli.py --spec \
Documentation/netlink/specs/netdev.yaml --dump qstats-get --json \
'{"ifindex":14, "scope":"queue"}'

[{'ifindex': 14, 'queue-id': 0, 'queue-type': 'rx', 'rx-bytes': 758,
'rx-hw-drop-overruns': 0, 'rx-hw-drops': 0, 'rx-packets': 11},
 {'ifindex': 14, 'queue-id': 1, 'queue-type': 'rx', 'rx-bytes': 0,
'rx-hw-drop-overruns': 0, 'rx-hw-drops': 0, 'rx-packets': 0},
{'ifindex': 14, 'queue-id': 0, 'queue-type': 'tx', 'tx-bytes': 0,
'tx-hw-drop-errors': 0, 'tx-hw-drops': 0, 'tx-packets': 0},
 {'ifindex': 14, 'queue-id': 1, 'queue-type': 'tx', 'tx-bytes': 0,
'tx-hw-drop-errors': 0, 'tx-hw-drops': 0, 'tx-packets': 0},
 {'ifindex': 14, 'queue-id': 2, 'queue-type': 'tx', 'tx-bytes': 810,
'tx-hw-drop-errors': 0, 'tx-hw-drops': 0, 'tx-packets': 10},]

Signed-off-by: Bhargava Marreddy <bhargava.marreddy@broadcom.com>
Reviewed-by: Vikas Gupta <vikas.gupta@broadcom.com>
Link: https://patch.msgid.link/20260406180420.279470-10-bhargava.marreddy@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>