net: marvell: clean up trigraph warning on ??! string

The character sequence ??! is a trigraph and causes the following
clang warning:

drivers/net/ethernet/marvell/mvneta.c:2604:39: warning: trigraph ignored [-Wtrigraphs]

Clean this by replacing it with single ?.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>