RDMA/uverbs: Fix error cleanup path of ib_uverbs_add_one()

If ib_uverbs_create_uapi() fails, dev_num should be freed from the bitmap.

Fixes: 7d96c9b1 ("IB/uverbs: Have the core code create the uverbs_root_spec")
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>