RDMA/restrack: clean up res_to_dev()

Simplify res_to_dev() to make it easier to read/maintain.

Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>