iio: mma7455: Use scan_type when processing raw data

Use channel definition as root of trust and replace constant
when reading elements directly using the raw sysfs attributes.

Signed-off-by: Gwendal Grignou <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/accel/mma7455_core.c

···
 		if (ret)
 			return ret;
 
-		*val = sign_extend32(le16_to_cpu(data), 9);
+		*val = sign_extend32(le16_to_cpu(data),
+				     chan->scan_type.realbits - 1);
 
 		return IIO_VAL_INT;