usb: dwc2: pci: Move devm_kzalloc() before platform_device_add()

After platform_device_add(), if we error out, we must do
platform_device_unregister(), which also does the put. So lets move
devm_kzalloc() to simplify error handling and avoid calling of
platform_device_unregister().

Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Vardan Mikayelyan <mvardan@synopsys.com>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Grigor Tovmasyan <tovmasya@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>