wifi: mac80211: add tx_handlers_drop statistics to ethtool

wifi: mac80211: add tx_handlers_drop statistics to ethtool

Currently tx_handlers_drop statistics are handled only for slow TX
path and only at radio level. This also requires
CONFIG_MAC80211_DEBUG_COUNTERS to be enabled to account the dropped
packets. There is no way to check these stats for fast TX,
at interface level and monitor without enabling the debug configuration.

Hence, add a new counter at the sdata level to track packets dropped
with reason as TX_DROP during transmission for fast path, slow path
and other tx management packets. Expose this via ethtool statistics,
to improve visibility into transmission failures at interface level
and aid debugging and performance monitoring.

Place the counter in ethtool with other available tx_* stats for
better readability and accurate tracking.

Sample output:
root@buildroot:~# ethtool -S wlan0
NIC statistics:
     rx_packets: 5904
     rx_bytes: 508122
     rx_duplicates: 12
     rx_fragments: 5900
     rx_dropped: 12
     tx_packets: 391487
     tx_bytes: 600423383
     tx_filtered: 0
     tx_retry_failed: 10332
     tx_retries: 1548
     tx_handlers_drop: 4
     ....

Co-developed-by: Hari Chandrakanthan <quic_haric@quicinc.com>
Signed-off-by: Hari Chandrakanthan <quic_haric@quicinc.com>
Signed-off-by: Sarika Sharma <quic_sarishar@quicinc.com>
Link: https://patch.msgid.link/20250822052110.513804-1-quic_sarishar@quicinc.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>