regulator: max8660: Fix a memory leak due to missing devm_kzalloc conversion

commit 4d26f7 "regulator: max8660: Use devm_kzalloc()" missed to
replace kzalloc by devm_kzalloc. Fix it.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>