net/mlx5e: Fix wrong code indentation in conditional statement

Fix the following checkpatch warning in en_ethtool.c:
WARNING: suspect code indent for conditional statements (8, 9)
+	for (i = 0; i < NUM_PCIE_PERF_STALL_COUNTERS(priv); i++)
+	 strcpy(data + (idx++) * ETH_GSTRING_LEN,

Signed-off-by: Gal Pressman <galp@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>