bridge: fix STP roles and protos strings

Add the missing commas that got lost in e5539fb618cc7.

PR: 252532
Reviewd by: kp@, donner@, freqlabs@
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D28425

(cherry picked from commit 88be0e11205e4a2dc77e29f7b4a2e82b8d7c9a5c)