cam iosched: Use the existing CTLFLAG_RDTUN and CTLFLAG_RWTUN flag definitions

Use them when possible, instead of separated flags.

No functional change intended.

Reviewed by: hselasky, erj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D39466

(cherry picked from commit 69cb72b8728ba605d6cc8758480eca3fc6fd2ed6)
(cherry picked from commit 09f9dad4654af6821f8d9e70d35a8cc78d1305fe)