USB: serial: ftdi_sio: report the valid GPIO lines to gpiolib

Since it is pretty common for only some of the CBUS lines to be
valid as GPIO lines, let's report such validity to the rest of
the kernel.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201204164739.781812-3-maz@kernel.org
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Johan Hovold <johan@kernel.org>