clocksource/drivers/pistachio: Fix trivial typo

Fix trivial typo, rename local variable from 'overflw' to 'overflow' in
pistachio_clocksource_read_cycles().

Reported-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Drew Fustini <drew@beagleboard.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210305090315.384547-1-drew@beagleboard.org