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.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>