clocksource/drivers/pistachio: Fix -Wunused-but-set-variable warning

Variable 'overflow' set but not used, but this is the intended behavior.
The hardware only updates the counter register after the overflow
register read. However, the value of overflow is not actually needed.

Link: https://lore.kernel.org/lkml/202111200402.afQsussU-lkp@intel.com/
Reported-by: kernel test robot <lkp@intel.com>
Suggested-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Drew Fustini <dfustini@baylibre.com>
Link: https://lore.kernel.org/r/20211123192524.1038304-1-dfustini@baylibre.com
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

drivers/clocksource/timer-pistachio.c

@@ -71,7 +71,8 @@ static u64 notrace
 pistachio_clocksource_read_cycles(struct clocksource *cs)
 {
 	struct pistachio_clocksource *pcs = to_pistachio_clocksource(cs);
-	u32 counter, overflow;
+	__maybe_unused u32 overflow;
+	u32 counter;
 	unsigned long flags;
 
 	/*