charger-manager: Fix regulator_get() return check

This patch fixes return value checking of regulator_get() in
charger-manager driver. The API, regulator_get(), returns ERR_PTR() when
it fails to get regulator with given name, not NULL.

Signed-off-by: Jonghwa Lee <jonghwa3.lee@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>