LinuxKPI: const argument to irq_set_affinity_hint()

irq_set_affinity_hint() takes a const mask argument and some drivers
pass it in as such where earlier implementations were more lenient.
Deal with it and __DECONST() the argument when passed to intr_setaffinity().

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D38242