USB: serial: cp210x: provide gpio valid mask

Use the new GPIO valid-mask feature to inform gpiolib which pins are
available for use instead of handling that in a request callback.

This also allows user space to figure out which pins are available
through the chardev interface without having to request each pin in
turn.

Note that the return value when requesting an unavailable pin will now
be -EINVAL instead of -ENODEV.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>