Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/mediatek/mt76/mt7615/testmode.c: In function mt7615_tm_set_tx_power
drivers/net/wireless/mediatek/mt76/mt7615/testmode.c:83:7: warning: variable ‘index’ set but not used [-Wunused-but-set-variable]=

commit 4f0bce1c ("mt76: mt7615: implement testmode support")
involved this unused variable, remove it.
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>