usb: dwc3-generic: Return early when there is no child node

usb: dwc3-generic: Return early when there is no child node

The current error check for device_find_first_child is not working as
expected, the documentation for device_find_first_child mention:

  @devp: Returns first child device, or NULL if none
  Return: 0

Change to return early when there is no child node to avoid any possible
null pointer dereference.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>