USB: serial: ftdi_sio: implement GPIO support for FT-X devices

This patch allows using the CBUS pins of FT-X devices as GPIO in CBUS
bitbanging mode. There is no conflict between the GPIO and VCP
functionality in this mode. Tested on FT230X and FT231X.

As there is no way to request the current CBUS register configuration
from the device, all CBUS pins are set to a known state when the first
GPIO is requested. This allows using libftdi to set the GPIO pins
before loading this module for UART functionality, a behavior that
existing applications might be relying upon (though no specific case
is known to the authors of this patch).

Signed-off-by: Karoly Pados <pados@pados.hu>
[ johan: minor style changes ]
Signed-off-by: Johan Hovold <johan@kernel.org>