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: a3e0b51 ("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>

Author: sknsean

drivers/iio/accel/fxls8962af-core.c

@@ -749,6 +749,7 @@ static const struct iio_event_spec fxls8962af_event[] = {
 			      BIT(IIO_CHAN_INFO_OFFSET),\
 	.scan_index = -1, \
 	.scan_type = { \
+		.sign = 's', \
 		.realbits = 8, \
 		.storagebits = 8, \
 	}, \