RDMA/cma: Fix error flow in default_roce_mode_store

In default_roce_mode_store(), we took a reference to cma_dev, but didn't
return it with cma_dev_put in the error flow.

Fixes: 1c15b4f2 ("RDMA/core: Modify enum ib_gid_type and enum rdma_network_type")
Link: https://lore.kernel.org/r/20210113130214.562108-1-leon@kernel.org


Signed-off-by: Neta Ostrovsky <netao@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>