mt76: mt7915: fix the muru tlv issue

The muru enable/disable are only set after the first station connection.
Without this patch, the firmware couldn't enable muru
if the first connected station is non-HE type.

Fixes: 16bff457 ("mt76: mt7915: rework mt7915_mcu_sta_muru_tlv()")
Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: MeiChia Chiu <meichia.chiu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>