cc2538: clock: Fix secs update in clock_adjust()

Whole elapsed seconds are added to secs first, so only the remaining subsecond
ticks should then be subtracted from second_countdown in order to decide whether
secs should be incremented again.

Otherwise, secs is not correctly updated in some cases, typically if the bit 7
of ticks is 1. E.g., with ticks = 128 (i.e. exactly 1 s elapsed) and
second_countdown = 128, secs was first incremented as expected, then 128 was
subtracted from second_countdown, giving 0 and triggering an unwanted second
increment of secs. Or with ticks = 129 (i.e. 1 s + 1 tick) and
second_countdown = 1, secs was first incremented as expected, then 129 was
subtracted from second_countdown, giving 128 and missing a second increment of
secs that should have occurred because second_countdown wrapped around.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>

cpu/cc2538/clock.c

@@ -188,7 +188,7 @@ clock_adjust(clock_time_t ticks)
    * Update internal second countdown so that next second change will actually
    * happen when it's meant to happen.
    */
-  second_countdown -= ticks;
+  second_countdown -= ticks & 127;
 
   if(second_countdown == 0 || second_countdown > 128) {
     secs++;