kern/intr: redeclare intr_setaffinity()'s third arg constant

This matches reality and allows removal of a __DECONST().

Fixes: 4c72d075a57 ("LinuxKPI: const argument to irq_set_affinity_hint()")
Fixes: 9b33b154b53 ("Add support to cpuset for binding hardware interrupts")
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1126