iio: accel: fxls8962af: Fix temperature scan element sign

iio: accel: fxls8962af: Fix temperature scan element sign

Mark the temperature element signed, data read from the TEMP_OUT register
is in two's complement format.
This will avoid the temperature being mishandled and miss displayed.

Fixes: a3e0b51884ee ("iio: accel: add support for FXLS8962AF/FXLS8964AF accelerometers")
Suggested-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Cc: stable@vger.kernel.org
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Signed-off-by: Sean Nyekjaer <sean@geanix.com>
Link: https://patch.msgid.link/20250505-fxls-v4-2-a38652e21738@geanix.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>