isp1704_charger: Correct length for storing model

Model should have room to accommodate the trailing null byte,
"isp170[4|7]\0".

Signed-off-by: Ameya Palande <ameya.palande@nokia.com>
Acked-by: Heikki Krogerus <heikki.krogerus@nokia.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>