net: stmmac: fix off-by-one error in sanity check

My previous patch had an off-by-one error in the added sanity
check, the arrays are MTL_MAX_{RX,TX}_QUEUES long, so if that
index is that number, it has overflown.

The patch silenced the warning anyway because the strings could
no longer overlap with the input, but they could still overlap
with other fields.

Fixes: 3e0d5699 ("net: stmmac: fix gcc-10 -Wrestrict warning")
Reported-by: Russell King (Oracle) <linux@armlinux.org.uk>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>