usb: cdc: Add unique PIDs for each sample

Unique PID is required for each sample in order
to be recognized by host.

When creating a new sample:
- Add USB_PID_<SAMPLE_NAME>_SAMPLE
  in samples/subsys/usb/usb_pid.Kconfig
- Create Kconfig file in your sample's subdirectory, containing:
  config USB_DEVICE_PID
	default USB_PID_<SAMPLE_NAME>_SAMPLE

  source "Kconfig.zephyr"

Signed-off-by: Marcin Szymczyk <Marcin.Szymczyk@nordicsemi.no>