net/mlx5e: Use ethtool_sprintf/puts() to fill selftests strings

Use ethtool_sprintf/puts() helper functions which handle the common
pattern of printing a string into the ethtool strings interface and
incrementing the string pointer by ETH_GSTRING_LEN.

The int return value in mlx5e_self_test_fill_strings() is not removed as
it is still used to return the number of selftests.
Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240402133043.56322-3-tariqt@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/mellanox/mlx5/core/en_selftest.c

@@ -359,7 +359,7 @@ int mlx5e_self_test_fill_strings(struct mlx5e_priv *priv, u8 *data)
 		if (st.cond_func && st.cond_func(priv))
 			continue;
 		if (data)
-			strcpy(data + count * ETH_GSTRING_LEN, st.name);
+			ethtool_puts(&data, st.name);
 		count++;
 	}
 	return count;