mt7615: mcu: simplify __mt7615_mcu_set_wtbl

Do not loop over cmd payload in __mt7615_mcu_set_wtbl since it is
already done in before running __mt7615_mcu_set_wtbl (e.g.
mt7615_mcu_set_wtbl_key)

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>