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

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>