hw/ds1338: Fix conversion between 12 hours and 24 hours modes.

The proper mapping between 24 hours and 12 hours modes is:
0       12 AM
1-11    1-11 AM
12      12 PM
13-23   1-11 PM
Fix code accordingly.

Signed-off-by: Antoine Mathys <barsamin@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/ds1338.c

diff --git a/hw/ds1338.c b/hw/ds1338.c
index 6f70538..1da0f96 100644
--- a/hw/ds1338.c
+++ b/hw/ds1338.c
@@ -59,8 +59,8 @@
     s->nvram[1] = to_bcd(now.tm_min);
     if (s->nvram[2] & HOURS_12) {
         int tmp = now.tm_hour;
-        if (tmp == 0) {
-            tmp = 24;
+        if (tmp % 12 == 0) {
+            tmp += 12;
         }
         if (tmp <= 12) {
             s->nvram[2] = HOURS_12 | to_bcd(tmp);
@@ -145,8 +145,8 @@
                 if (data & HOURS_PM) {
                     tmp += 12;
                 }
-                if (tmp == 24) {
-                    tmp = 0;
+                if (tmp % 12 == 0) {
+                    tmp -= 12;
                 }
                 now.tm_hour = tmp;
             } else {