arm64: arch_timer: Get rid of erratum_workaround_set_sne

Let's move the handling of workarounds affecting set_next_event
to the affected function, instead of overriding the pointers
as an afterthough. Yes, this is an extra indirection on the
erratum handling path, but the HW is busted anyway.

This will allow for some more flexibility later.

Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>