iio:magnetometer:ak8975: fix uninitialized chipset

ak_def_array bounds are not properly checked in case of ACPI matching
failure. GCC warns with the following message at line 799:
‘chipset’ may be used uninitialized in this function.

Signed-off-by: Gregor Boirie <gregor.boirie@parrot.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>