This fixes:
mt7603/dma.c: In function 'mt7603_rx_loopback_skb':
mt7603/dma.c:60:17: error: ISO C90 forbids mixed declarations and code [-Werror=declaration-after-statement]
Fixes: 2568b30285 ("mt76: backport mt7603 fixes important for its stability")
Fixes: https://github.com/openwrt/openwrt/issues/15510
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
