USB: serial: ftdi_sio: log the CBUS GPIO validity

The validity of the ftdi CBUS GPIO is pretty hidden so far,
and finding out *why* some GPIOs don't work is sometimes
hard to identify. So let's help the user by displaying the
map of the CBUS pins that are valid for a GPIO.

Suggested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201204164739.781812-4-maz@kernel.org
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
[johan: demote to KERN_DEBUG, rephrase messages, drop ftx-prog warning]
Signed-off-by: Johan Hovold <johan@kernel.org>