clocksource/drivers/arm_arch_timer: Use stable count reader in erratum sne

In commit 0ea41539 ("clocksource/arm_arch_timer: Use arch_timer_read_counter
to access stable counters"), we separate stable and normal count reader to omit
unnecessary overhead on systems that have no timer erratum.

However, in erratum_set_next_event_tval_generic(), count reader becomes normal
reader. This converts it to stable reader.

Fixes: 0ea41539 ("clocksource/arm_arch_timer: Use arch_timer_read_counter to access stable counters")
Acked-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20201204073126.6920-2-zhukeqian1@huawei.com