net: dsa: mv88e6xxx: fix broken if statement because of a stray semicolon

There is a stray semicolon in an if statement that will cause a dev_err
message to be printed unconditionally. Fix this by removing the stray
semicolon.

Addresses-Coverity: ("Stay semicolon")
Fixes: f0942e00 ("net: dsa: mv88e6xxx: Add support for port mirroring")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>