selftests/timers: fix write return value handlng

The function can return negative value.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2038576

Signed-off-by: Andrzej Hajda <[e-mail]>
Signed-off-by: John Stultz <[e-mail]>

tools/testing/selftests/timers/clocksource-switch.c

···
 int change_clocksource(char *clocksource)
 {
 	int fd;
-	size_t size;
+	ssize_t size;
 
 	fd = open("/sys/devices/system/clocksource/clocksource0/current_clocksource", O_WRONLY);