USB: serial: ftdi_sio: fix gpio name collisions

Drop the gpio line names, which cause gpiolib to complain loudly
whenever a second ftdi gpiochip is registered:

	gpio gpiochip5: Detected name collision for GPIO name 'CBUS0'
	gpio gpiochip5: Detected name collision for GPIO name 'CBUS1'
	gpio gpiochip5: Detected name collision for GPIO name 'CBUS2'
	gpio gpiochip5: Detected name collision for GPIO name 'CBUS3'

and also prevents the legacy sysfs interface from being used (as the
line names are used as device names whenever they are set):

	sysfs: cannot create duplicate filename '/class/gpio/CBUS0'

Until non-unique names are supported by gpiolib (without warnings and
stack dumps), let's leave the gpio lines unnamed.

Signed-off-by: Johan Hovold <johan@kernel.org>