RDMA/mlx5: Organize QP types checks in one place

RDMA/mlx5: Organize QP types checks in one place

Perform check if QP type is supported in one place at the beginning of
the create_qp function instead of current implementation with checks
buried inside of the code.

Link: https://lore.kernel.org/r/20200427154636.381474-2-leon@kernel.org
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>