ice(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 5ff8018108278d06e024676c539e9f60f8189e83)