CP2105 gives kernel error -22 when in modem mode

[Maarten Brock <m.brock@vanmierlo.com>]

Hello all,

When a uart of the CP2105 USB-serial chip is programmed to be in modem 
mode, all its gpio pins are in alternate use and none are available for 
gpio. Still the cp210x driver tries to add a gpiochip unconditionally, 
which results in an error.

> cp210x 1-1.4.4:1.0: cp210x converter detected
> usb 1-1.4.4: cp210x converter now attached to ttyUSB0
> cp210x 1-1.4.4:1.1: cp210x converter detected
> gpio gpiochip2: (cp210x): tried to insert a GPIO chip with zero lines
> gpiochip_add_data_with_key: GPIOs 0..-1 (cp210x) failed to register, 
> -22
> cp210x 1-1.4.4:1.1: GPIO initialisation failed: -22
> usb 1-1.4.4: cp210x converter now attached to ttyUSB1

I propose to add something like the following to cp210x_gpio_init

+	unsigned long valid_mask, altfunc_mask;
...
+	altfunc_mask = priv->gpio_altfunc;
+	bitmap_complement(&valid_mask, &altfunc_mask, priv->gc.ngpio);
+	if (bitmap_empty(&valid_mask, priv->gc.ngpio))
+		return 0;
+
  	priv->gc.label = "cp210x";
  	priv->gc.request = cp210x_gpio_request;
  	priv->gc.get_direction = cp210x_gpio_direction_get;

I can write a proper patch, but am unsure if and what the Fixes tag 
should be.

Maarten