serial: Set up the 'priv' pointer when creating a serial device

serial: Set up the 'priv' pointer when creating a serial device

The stdio_dev structure has a private pointer for its creator, but it is
not set up by the serial system. Set it to point to the serial device so
that it can be found by code called by stdio.

Signed-off-by: Simon Glass <sjg@chromium.org>