power_supply: isp1704_charger: Error messages when probe fail

This patch adds more detailed error messages when probe function fails.
It is useful for debbuging why the driver refuses to register the
charger device.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Sebastian Reichel <sre@kernel.org>