wifi: mt76: mt7915: fix mt7915_mac_set_timing()

Correct mac timiing settings for different hardware generations.
This improves 40-60Mbps performance.

Fixes: 9aac2969 ("mt76: mt7915: update mac timing settings")
Reported-By: Carson Vandegriffe <carson.vandegriffe@candelatech.com>
Tested-by: Chad Monroe <chad.monroe@smartrg.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>