ds1621: Fix negative temperature readings

Fix bug where signed data was processed as unsigned.  The bug previously
resulted in negative temperature readings wrapping around, eg -10 became
245.

Signed-off-by: Jeff Dischler <jdischler@xes-inc.com>
Signed-off-by: Peter Tyser <ptyser@xes-inc.com>

drivers/hwmon/ds1621.c

@@ -80,7 +80,7 @@ int dtt_read(int sensor, int reg)
 
 	/* Handle 2 byte result */
 	if (dlen == 2)
-		return ((int)((short)data[1] + (((short)data[0]) << 8)));
+		return (short)((data[0] << 8) | data[1]);
 
 	return (int)data[0];
 }