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
Approved by: re (cperciva)
Differential Revision: https://reviews.freebsd.org/D38242

(cherry picked from commit 4c72d075a57e9a3215639aede16df4df29e1c1da)
(cherry picked from commit 4724e1867e81ab5de2c6b5a6819998b660f3e96d)