power: charger-manager: Fix checking of wrong return type

This patch fix minor issue about checking wrong return type.

The of_cm_parse_desc() return ERR_PTR(errnor number) when some error happen
in this function. But, charger_manager_probe() has only checked whether
desc is NULL or not. If of_cm_parse_desc() returns ERR_PTR(-ENOMEM), desc
isn't NULL but desc is (void *)(-ENOMEM). Althouhg some error happen for parsing
DT, charger_manager_probe() can't detect error of desc instance.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Myungjoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>