i2c-mux-gpio: Use devm_kzalloc instead of kzalloc

Use the devm_kzalloc managed function to stripdown the error and remove
code.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Jean Delvare <khali@linux-fr.org>