cam/scsi: Add sysctl flag CTLFLAG_TUN to loader tunable

The sysctl variable 'kern.cam.scsi_delay' is actually a loader tunable.
Add sysctl flag CTLFLAG_TUN to it so that sysctl -T will report it
correctly.

No functional change intended.

Reviewed by: kib, imp (for cam)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42113

(cherry picked from commit e2ad7ce37b1a354750d2dd3567e77f72edeb7e1d)