Replace kzalloc with the device managed devm_kzalloc

Cc: Alexandre Courbot <gnurou@gmail.com>
Cc: John Crispin <blogic@openwrt.org>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>