wifi: mt76: mt7996: exclude tx backoff time from airtime

This helps to make STA airtime counters more accurate.

Signed-off-by: Chad Monroe <chad@monroe.io>
Suggested-by: Ryder Lee <ryder.lee@mediatek.com>
Link: https://patch.msgid.link/88821d8f08e2715927939b35e630a06171962636.1732653612.git.chad@monroe.io


Signed-off-by: Felix Fietkau <nbd@nbd.name>