gpio-charger: Check result of kzalloc

Since kzalloc can return NULL we have to check its result.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>
Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>