net: stmmac: remove unused variable in sysfs_display_ring

The u64 x variable in sysfs_display_ring is unused.
This patch remove it.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>