usb: device_next: usbd_hid: Fix size in HID report get

Since UDC buffers are allocated with `UDC_BUF_GRANULARITY` granularity,
the `net_buf_tailroom` may no longer be equal to the HID report size.
Use `setup->wLength` instead to ensure that proper HID report size is
passed to the application's callback.

Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>