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.
This is needed for future wireless driver updates.
Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Obtained from: bz_git_iwlwifi (Dec 2020) (partly)