mt76: mt7915: rework .set_bitrate_mask() to support more options

With this patch, driver can support single rate, (HE)GI and HE_LTF
configuration through .set_bitrate_mask().

Tested-by: MeiChia Chiu <meichia.chiu@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>