mt76: mt7915: update mac timing settings

1. EIFS has been divided into OFDM/CCK fields after 11ax generation.
2. For 5G/6G SIFS setting, hardware counts extra 6us for OFDM.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>