extcon: max77693: Use devm_kzalloc

devm_kzalloc() is a device managed function. It makes error handling
and cleanup code a bit simpler.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>