wifi: mt76: mt7915: fix tlv length of mt7915_mcu_get_chan_mib_info

Correct per-device TLV lengths to avoid invalid operation in firmware.
(  64.040375:28:STATS-E)statsGetSingleHWCounter: MIB counter index = 65472 not supported.
This happens on mt7916/mt7986.

Fixes: b0bfa005 ("wifi: mt76: mt7915: improve accuracy of time_busy calculation")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>