power: supply: bq2415x: check for NULL acpi_id to avoid null pointer dereference

acpi_match_device can potentially return NULL, so it is prudent to
check if acpi_id is null before it is dereferenced.  Add a check
and an error message to indicate the failure.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>