net/mlx5e: Remove redundant xsk pointer check in mlx5e_mpwrq_validate_xsk

This validation function is relevant only for XSK cases, hence it
assumes to be called only with xsk != NULL.
Thus checking for invalid xsk pointer is redundant and misleads static
code analyzers.
This commit removes redundant xsk pointer check.

This solves the following smatch warning:
drivers/net/ethernet/mellanox/mlx5/core/en/params.c:481
mlx5e_mpwrq_validate_xsk() error: we previously assumed 'xsk' could be
null (see line 478)

Fixes: 6470d2e7 ("net/mlx5e: xsk: Use KSM for unaligned XSK")
Signed-off-by: Adham Faris <afaris@nvidia.com>
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>