net/mlx5e: Check return value of snprintf writing to fw_version buffer

Treat the operation as an error case when the return value is equivalent to
the size of the name buffer. Failed to write null terminator to the name
buffer, making the string malformed and should not be used. Provide a
string with only the firmware version when forming the string with the
board id fails.

Without check, will trigger -Wformat-truncation with W=1.

    drivers/net/ethernet/mellanox/mlx5/core/en_ethtool.c: In function 'mlx5e_ethtool_get_drvinfo':
    drivers/net/ethernet/mellanox/mlx5/core/en_ethtool.c:49:31: warning: '%.16s' directive output may be truncated writing up to 16 bytes into a region of size between 13 and 22 [-Wformat-truncation=]
      49 |                  "%d.%d.%04d (%.16s)",
         |                               ^~~~~
    drivers/net/ethernet/mellanox/mlx5/core/en_ethtool.c:48:9: note: 'snprintf' output between 12 and 37 bytes into a destination of size 32
      48 |         snprintf(drvinfo->fw_version, sizeof(drvinfo->fw_version),
         |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      49 |                  "%d.%d.%04d (%.16s)",
         |                  ~~~~~~~~~~~~~~~~~~~~~
      50 |                  fw_rev_maj(mdev), fw_rev_min(mdev), fw_rev_sub(mdev),
         |                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      51 |                  mdev->board_id);
         |                  ~~~~~~~~~~~~~~~

Fixes: 84e11edb ("net/mlx5e: Show board id in ethtool driver information")
Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6d4ab2e97dcfbcd748ae71761a9d8e5e41cc732c


Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>