net/mlx5: Fix an IS_ERR() vs NULL bug in esw_qos_move_node()

The __esw_qos_alloc_node() function returns NULL on error.  It doesn't
return error pointers.  Update the error checking to match.

Fixes: 96619c48 ("net/mlx5: Add support for setting tc-bw on nodes")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/0ce4ec2a-2b5d-4652-9638-e715a99902a7@sabinyo.mountain


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