powerpc: 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 dc1c5138c3255cc9004a3842001ed6c5db8f4ea3)