rtadvd(8): Fix RA flag inconsistency messages

During flag inconsistency report, we handle rai->rai_otherflg
as a bool, but the value is 0x40. Make it a simple number comparison.

PR: 295995
Reviewed by: markj, Faraz Vahedi <kfv@kfv.io>
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D58672