iicbus(4): Use the existing CTLFLAG_RWTUN flag definition

Use it 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 deac4c7f0753841de8da4d7f37d7e49620b3bbb4)