iio: ak8975: fix maybe-uninitialized warning

If i2c_device_id *id is NULL and acpi_match_device returns NULL too,
then chipset may be unitialized when accessing &ak_def_array[chipset] in
ak8975_probe. Therefore initialize chipset to AK_MAX_TYPE, which will
return an error when not changed.

This patch fixes the following maybe-uninitialized warning:

drivers/iio/magnetometer/ak8975.c: In function ‘ak8975_probe’:
drivers/iio/magnetometer/ak8975.c:788:14: warning: ‘chipset’ may be used
uninitialized in this function [-Wmaybe-uninitialized]
  data->def = &ak_def_array[chipset];

Signed-off-by: Richard Leitner <dev@g0hl1n.net>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>