net: airoha: fix MIB stats collection to be lossless

net: airoha: fix MIB stats collection to be lossless

REG_FE_GDM_MIB_CLEAR after every read creates a race window where
packets arriving between read and clear are lost from statistics.

Switch to a delta-based approach instead:

- 64-bit H+L registers (ok pkts/bytes, E64..L1023): read absolute
  hardware total directly into a local variable; clamp with max(new, old)
  to prevent torn-read regression when the counter carries between the
  two reads.

- 32-bit registers (drops, bc, mc, errors, runt, long): accumulate
  (u32)(curr - prev) into a 64-bit software counter; unsigned
  subtraction handles wrap-around transparently.

- tx/rx_len[0] ([0,64] bucket): combines RUNT_CNT (32-bit, delta via
  tx_runt/rx_runt) and E64_CNT (64-bit, absolute) into a single local
  accumulator; max(new, old) applied here too to guard against a torn
  read of E64 when the RUNT accumulator is unchanged between polls.

MIB counters are zeroed by the SCU FE reset (EN7581_FE_RST) asserted
in airoha_hw_init() at module load, so no explicit MIB clear is needed
in airoha_fe_init().

Merge airoha_dev_get_hw_stats() into airoha_update_hw_stats() and
move stats_lock inside. Plain spin_lock() is correct: the function
is only called from ndo_get_stats64() in process context. Each dev
refreshes only its own MIB counters; sibling devs on a shared GDM3/4
port are polled when their own netdev is queried.

Fixes: 8f4695fb67b2 ("net: airoha: better handle MIBs for GDM ports with multiple devs attached")
Signed-off-by: Aniket Negi <aniket.negi03@gmail.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20260707152639.105628-1-aniket.negi03@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>