i2c: i801: Improve i801_add_mux

The return value of i801_add_mux() isn't used, so let's change it to void.
In addition remove the not needed cast to struct gpiod_lookup.
GPIO_LOOKUP() uses GPIO_LOOKUP_IDX() that includes this cast.

Reviewed-by: Jean Delvare <jdelvare@suse.de>
Tested-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>