RDMA/mlx5: Globally parse DEVX UID

Remove duplication in parsing of DEVX UID.

Link: https://lore.kernel.org/r/20200427154636.381474-27-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>