power: bq24735-charger: Assume not charging when charger is missing

When the charger is missing (disconnected), it is safe to assume that
the charger chip is no charging.

This is especially relevant when a status GPIO is present and the
charger is getting disconnected. bq24735_charger_is_charging will be
triggered due to the interrupt then, it will attempt to read whether it
is charging through i2c, which will fail as the charger is disconnected.

This also fixes that specific issue.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Signed-off-by: Sebastian Reichel <sre@kernel.org>