w1: w1 temp: fix negative termperature calculation

Fix regression caused by commit 507e2fb
("w1: w1 temp calculation overflow fix") whereby negative temperatures for
the DS18B20 are not converted properly.

When the temperature exceeds 32767 milli-degrees the temperature overflows
to -32768 millidegrees.  These are both well within the -55 - +125 degree
range for the sensor.

Addresses https://bugzilla.kernel.org/show_bug.cgi?id=12646

Signed-of-by: Ian Dall <ian@beware.dropbear.id.au>
Cc: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
Tested-by: Karsten Elfenbein <kelfe@gmx.de>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/w1/slaves/w1_therm.c

@@ -115,9 +115,8 @@ static struct w1_therm_family_converter w1_therm_families[] = {
 
 static inline int w1_DS18B20_convert_temp(u8 rom[9])
 {
-	int t = ((s16)rom[1] << 8) | rom[0];
-	t = t*1000/16;
-	return t;
+	s16 t = le16_to_cpup((__le16 *)rom);
+	return t*1000/16;
 }
 
 static inline int w1_DS18S20_convert_temp(u8 rom[9])