wifi: mt76: mt7915: fix possible integer overflows in mt7915_muru_stats_show()

wifi: mt76: mt7915: fix possible integer overflows in mt7915_muru_stats_show()

Assuming sums of values stored in variables such as sub_total_cnt
and total_ppdu_cnt are big enough to warrant their u64 type, it
makes sense to ensure that their calculation takes into account
possible integer overflow issues.

Play it safe and fix the problem by casting right hand expressions
to u64 as well. Also, slightly adjust tabulation.

Found by Linux Verification Center (linuxtesting.org) with static
analysis tool SVACE.

Fixes: 1966a5078f2d ("mt76: mt7915: add mu-mimo and ofdma debugfs knobs")
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@fintech.ru>
Link: https://patch.msgid.link/20250114154441.16920-1-n.zhandarovich@fintech.ru
Signed-off-by: Felix Fietkau <nbd@nbd.name>