net: stmmac: Pad ring number with zeroes in display_ring()

Make the printing of the ring number consistent and properly aligned by
padding the ring number with up to 3 zeroes, which covers the maximum
ring size. This makes it a lot easier to see outliers in debug prints.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>