usb: dwc3: core: Avoid redundant system suspend/resume callbacks

usb: dwc3: core: Avoid redundant system suspend/resume callbacks

dwc3 device suspend/resume callbacks were being triggered during system
suspend and resume even if the device was already runtime-suspended.
This is redundant for device mode because the suspend and resume routines
are essentially identical for system PM and runtime PM.

To prevent these unnecessary callbacks, indicate to the PM core that it
can safely leave the device in runtime suspend if it's already
runtime-suspended in device mode by returning a positive value in
prepare() callback. This optimization only applies to devices without
pinctrl, as pinctrl has distinct logic tied to system suspend/resume.

Signed-off-by: Roy Luo <royluo@google.com>
Acked-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/20250312223434.3071598-1-royluo@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>