bq2415x_charger: Fix null pointer dereference

Commit b68c3161 (bq2415x_charger: Allow to load and use driver even if
notify device is not registered yet) introduced null pointer dereference in
case bq is NULL. This patch fixes it.

Fixes: b68c3161 ("bq2415x_charger: Allow to load and use driver even if notify device is not registered yet")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>