power: supply: charger-manager: Verify polling interval only when polling requested

Driver bails out with -EINVAL when no polling specififaion is requested.
Fix that by verifing polling interval only if polling_mode is different
from CM_POLL_DISABLE.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>