LinuxKPI: move pm_message_t from kernel.h to pm.h

Move pm_message_t from kernel.h to pm.h and remove a private define
in usb.h as well as adjust the implementation in linux_usb.c.
This cleans up what I believe to be a historic shortcut and is
needed for future wireless driver updates.

Leave a note in UPDATING that drm-kmod users need to update to the
latest version before re-compiling a new kernel to avoid errors
(see PR).

Sponsored by: The FreeBSD Foundation
PR: 264449 (drm-kmod port update, thanks wulf)
Obtained from: bz_git_iwlwifi (Dec 2020) (partly)
Reviewed by: hselasky, imp
Differential Revision: https://reviews.freebsd.org/D35276

(cherry picked from commit 0e981d79b19da8fc550ea0dbe46e8658be988db4)