usb: device_next: hid: fix Get Report buffer handling

After the get_report() callback, we need to determine how many bytes the
HID device wrote to the report buffer. Use the callback return value to
do this, and modify the net_buf data length value if get_report was
successful.

Reported-by: Marek Pieta <Marek.Pieta@nordicsemi.no>
Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>