mt76: mt7615: check return value of mt7615_eeprom_get_power_index

mt7615_eeprom_get_power_index can return negative error value.
Check mt7615_eeprom_get_power_index return value before using it

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