RDMA/mlx5: Process create QP flags in one place

create_flags is checked in too many places and scattered across all
the code, consolidate all the checks inside one function, so we will
be easily see the flow. As part of such change, delete unreachable code,
because IB/core is responsible sanitize the input.

Link: https://lore.kernel.org/r/20200427154636.381474-15-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>