drivers: net: xgene: fix kbuild warnings

Fixed the following kbuild warnings:
1. unused variable 'of_id'
2. buffer overflow 'ring_cfg' 5 <= 5

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>