Replace `volatile` access to pm_blocker by guarding the accesses with
`irq_disable()` ... `irq_restore()`.
`volatile` does only guarantee that no compiler optimizations are performed on
a variable access, but does not provide atomic access. E.g. on systems with
a memory bus of less than 32 bit, the access to pm_blocker cannot be done
with a single CPU instruction. Thus, resorting to disabling IRQs is the easiest
and most portable way to actually achieve atomic access.
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