mlxsw: spectrum_ethtool: Fix -Wformat-truncation warning

Ethtool stats strings cannot be longer than 32 characters
('ETH_GSTRING_LEN'), including the terminating null byte. The format
string '%.29s_%.1d' can exceed this limitation if the per-TC counter
name exceeds 28 characters. Together with the underscore, the two digits
of the TC (bounded at 16) and the terminating null byte, more than 32
characters will be used.

Fix this by bounding the counter name at 28 characters which suppresses
the following build warning [1]. This does not affect ethtool output
since the longest counter name does not exceed this limitation.

[1]
drivers/net/ethernet/mellanox/mlxsw/spectrum_ethtool.c: In function ‘mlxsw_sp_port_get_strings’:
drivers/net/ethernet/mellanox/mlxsw/spectrum_ethtool.c:622:58: error: ‘snprintf’ output may be truncated before the last format character [-Werror=format-truncation=]
  622 |                 snprintf(*p, ETH_GSTRING_LEN, "%.29s_%.1d",
      |                                                          ^
In function ‘mlxsw_sp_port_get_tc_strings’,
    inlined from ‘mlxsw_sp_port_get_strings’ at drivers/net/ethernet/mellanox/mlxsw/spectrum_ethtool.c:677:4:
drivers/net/ethernet/mellanox/mlxsw/spectrum_ethtool.c:622:17: note: ‘snprintf’ output between 3 and 33 bytes into a destination of size 32
  622 |                 snprintf(*p, ETH_GSTRING_LEN, "%.29s_%.1d",
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  623 |                          mlxsw_sp_port_hw_tc_stats[i].str, tc);
      |                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/ethernet/mellanox/mlxsw/spectrum_ethtool.c: In function ‘mlxsw_sp_port_get_strings’:
drivers/net/ethernet/mellanox/mlxsw/spectrum_ethtool.c:622:58: error: ‘snprintf’ output may be truncated before the last format character [-Werror=format-truncation=]
  622 |                 snprintf(*p, ETH_GSTRING_LEN, "%.29s_%.1d",
      |                                                          ^
In function ‘mlxsw_sp_port_get_tc_strings’,
    inlined from ‘mlxsw_sp_port_get_strings’ at drivers/net/ethernet/mellanox/mlxsw/spectrum_ethtool.c:677:4:
drivers/net/ethernet/mellanox/mlxsw/spectrum_ethtool.c:622:17: note: ‘snprintf’ output between 3 and 33 bytes into a destination of size 32
  622 |                 snprintf(*p, ETH_GSTRING_LEN, "%.29s_%.1d",
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  623 |                          mlxsw_sp_port_hw_tc_stats[i].str, tc);
      |                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Simon Horman <horms@kernel.org> # build-tested
Signed-off-by: Paolo Abeni <pabeni@redhat.com>