amd-xgbe: fix potential memory leak in xgbe-debugfs

Added kfree() to avoid the memory leak when debugfs_create_dir() fails.

Signed-off-by: Geliang Tang <geliangtang@163.com>
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>