dmaengine: qcom_hidma: avoid freeing an uninitialized pointer

If device_node np doesn't contain child or first child doesn't have
property "reg" then hidma_mgmt_of_populate_channels() perfoms
deallocation on uninitialized local variable res.

The patch adds res initialization by NULL.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Anton Vasilyev <vasilyev@ispras.ru>
Reviewed-by: Sinan Kaya <okaya@codeaurora.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>