IB/mlx4: Remove unneeded NULL check

NULL check for kfree is unnecessary, remove it.

Fixes: b42dde47 ("IB/mlx4: Rework special QP creation error path")
Signed-off-by: Yuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>