mtd: ubi: fix unreleased fwnode_handle in find_volume_fwnode()

The 'fw_vols' fwnode_handle initialized via
device_get_named_child_node() requires explicit calls to
fwnode_handle_put() when the variable is no longer required.

Add the missing calls to fwnode_handle_put() before the function
returns.

Cc: stable@vger.kernel.org
Fixes: 51932f9f ("mtd: ubi: populate ubi volume fwnode")
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>