net/mlx5e: Use unsigned for mlx5e_get_max_num_channels

net/mlx5e: Use unsigned for mlx5e_get_max_num_channels

The max number of channels is always an unsigned int, use the correct
type to fix compilation errors done with strict type checking, e.g.:

error: call to ‘__compiletime_assert_1110’ declared with attribute
  error: min(mlx5e_get_devlink_param_num_doorbells(mdev),
  mlx5e_get_max_num_channels(mdev)) signedness error

Fixes: 74a8dadac17e ("net/mlx5e: Preparations for supporting larger number of channels")
Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Jacob Keller <Jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20260218072904.1764634-7-tariqt@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>