mt76: mt7915: use mt7915_mcu_get_txpower_sku() to get per-rate txpower

Get per-rate txpower with mcu command. This is the preparation of
co-driver for the next chipset, which has different tmac power registers
but can share this same command.

Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>