mt76: mt7915: remove mt7915_mcu_add_he()

Make mt7915_mcu_sta_he_tlv() as a part of mt7915_mcu_add_rate_ctrl() as
firmware rate control should get HE rate information from sta_rec_he,
and reduce a global function accordingly.

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