i2c-i801: Let i2c-mux-gpio find the GPIO chip

Now that i2c-mux-gpio is able to find the GPIO chip by itself, we can
delegate this task. The great thing here is that i2c-mux-gpio can
defer device probing until the gpio chip is available, so we no longer
depend on the module loading order.

Signed-off-by: Jean Delvare <khali@linux-fr.org>