dmaengine: dw: fix potential memory leak in dw_dma_parse_dt()

If the "dma-channels" DT property is missing, the dw_dma_parse_dt()
function return NULL, but not before allocating memory for a struct
dw_dma_platform_data through devres.  If the device supports parameter
detection, the probe still succeeds and the allocated memory is not
released until the device is removed.

Fix this by deferring the allocation until after checking the
"dma-channels" property.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>