clocksource/drivers/timer-rtl-otto: Drop set_counter function

The current counter value is a read only register. It will be
reset when writing a new target timer value with rttm_set_period().
rttm_set_counter() is essentially a noop. Drop it.

While this makes rttm_start_timer() and rttm_enable_timer() the
same functions keep both to make the established abstraction layers
for register and control functions active.

Downstream has already tested and confirmed a patch. See
https://github.com/openwrt/openwrt/pull/19468
https://forum.openwrt.org/t/support-for-rtl838x-based-managed-switches/57875/3788



Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Tested-by: Stephen Howell <howels@allthatwemight.be>
Tested-by: Bjørn Mork <bjorn@mork.no>
Link: https://lore.kernel.org/r/20250804080328.2609287-3-markus.stockhausen@gmx.de