power: supply: mt6370: Fix return value check in mt6370_chg_probe()

If create_singlethread_workqueue() fails, it returns a null pointer,
replace IS_ERR() check with NULL pointer check.

Fixes: 233cb8a4 ("power: supply: mt6370: Add MediaTek MT6370 charger driver")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: ChiaEn Wu <chiaen_wu@richtek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>