mt76: Remove set but not used variable 'idx'

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

drivers/net/wireless/mediatek/mt76/dma.c: In function mt76_dma_rx_fill:
drivers/net/wireless/mediatek/mt76/dma.c:377:6: warning: variable idx set but not used [-Wunused-but-set-variable]

It is not used since commit 17f1de56

 ("mt76:
add common code shared between multiple chipsets")

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>