clocksource/drivers/em_sti: Remove unneeded memset()s

Memory for cs and ced fields in struct em_sti_priv is allocated
by devm_kzalloc() in the beginning of em_sti_probe() so they
don't need to be zeroed one more time in
em_sti_register_clocksource() and in em_sti_register_clockevent().

Signed-off-by: Alexey Klimov <alexey.klimov@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>