iio: inv_mpu6050: Fix use-after-free in ACPI code

In some cases this can result in incorrectly returning a negative value
from asus_acpi_get_sensor_info and the AK8963 magnetometer failing to
show up.

Note cpm is an alias for buffer.pointer which isn't apparent in this
patch on it's own.

Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Crestez Dan Leonard <leonard.crestez@intel.com>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>