rtc: rtc_time_to_tm: use unsigned arithmetic

The input argument to rtc_time_to_tm() is unsigned as well as are members of
the output structure.  However signed arithmetic is used within for
calculations leading to incorrect results for input values outside the signed
positive range.  If this happens the time of day returned is out of range.

Found the problem when fiddling with the RTC and the driver where year was set
to an unexpectedly large value like 2070, e.g.:

rtc0: setting system clock to 2070-01-01 1193046:71582832:26 UTC (3155760954)

while it should be:

rtc0: setting system clock to 2070-01-01 00:15:54 UTC (3155760954)

Changing types to unsigned fixes the problem.

[akpm@linux-foundation.org: remove old-fashioned `register' keyword]
Signed-off-by: NMaciej W. Rozycki <macro@linux-mips.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: David Brownell <david-b@pacbell.net>
Cc: Dmitri Vorobiev <dmitri.vorobiev@gmail.com>
Cc: <stable@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/rtc/rtc-lib.c

@@ -51,7 +51,7 @@ EXPORT_SYMBOL(rtc_year_days);
  */
 void rtc_time_to_tm(unsigned long time, struct rtc_time *tm)
 {
-	register int days, month, year;
+	unsigned int days, month, year;
 
 	days = time / 86400;
 	time -= days * 86400;