net: dsa: mt7530: fix macro MIRROR_PORT

The inner pair of parentheses should be around the variable x

Fixes: 37feab60 ("net: dsa: mt7530: add support for port mirroring")
Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>