staging:iio:magnetometer:ak8975: Fix probe() error-handling

Fix ak8975_probe() to jump to the appropriate exit labels when an error
occurs. With the previous code, some cleanup actions were being skipped
for some error conditions.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Andrew Chew <achew@nvidia.com>
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>