W1: w1_therm.c is flagging 0C etc as invalid

The extra rom[0] check is flagging valid temperatures as invalid when
there is already a CRC data transmission check.

w1_therm_read_bin()
	if (rom[8] == crc && rom[0])
		verdict = 1;

Requiring rom[0] to be non-zero will flag as invalid temperature
conversions when the low byte is zero, specifically the temperatures 0C,
16C, 32C, 48C, -16C, -32C, and -48C.

The CRC check is produced on the device for the previous 8 bytes and is
required to ensure the data integrity in transmission.  I don't see why the
extra check for rom[0] being non-zero is in there.  Evgeniy Polyakov didn't
know either.  Just for a check I unplugged the sensor, executed a
temperature conversion, and read the results.  The read was all ff's, which
also failed the CRC, so it doesn't need to protect against a disconnected
sensor.

I have more extensive patches in the work, but these two trivial ones will
do for today.  I would like to hear from people who use the ds2490 USB to
one wire dongle.  1 if you would be willing to test the patches as I
currently only have the one sensor on a short parisite powered wire, 2 if
there is any cheap sources for the ds2490.
Signed-off-by: NDavid Fries <david@fries.net>
Acked-by: NEvgeniy Polyakov <johnpol@2ka.mipt.ru>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/w1/slaves/w1_therm.c

@@ -204,7 +204,7 @@ static ssize_t w1_therm_read_bin(struct kobject *kobj,
 
 				crc = w1_calc_crc8(rom, 8);
 
-				if (rom[8] == crc && rom[0])
+				if (rom[8] == crc)
 					verdict = 1;
 			}
 		}