mt76: mt7921: fix Wformat build warning

ARRAY_SIZE() is of type size_t, so the format specfier should
be %zu instead of %lu.

Fixes this build warning:

../drivers/net/wireless/mediatek/mt76/mt7921/main.c: In function ‘mt7921_get_et_stats’:
../drivers/net/wireless/mediatek/mt76/mt7921/main.c:1024:26: warning: format ‘%lu’ expects argument of type ‘long unsigned int’, but argument 4 has type ‘unsigned int’ [-Wformat=]
   dev_err(dev->mt76.dev, "ei: %d  SSTATS_LEN: %lu",

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Sean Wang <sean.wang@mediatek.com>
Cc: linux-wireless@vger.kernel.org
Cc: Felix Fietkau <nbd@nbd.name>
Cc: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Cc: Ryder Lee <ryder.lee@mediatek.com>
Cc: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211022233251.29987-1-rdunlap@infradead.org