bluetooth: mesh: brg_cfg_cli: Initialize prohibited value

Coverity complains about uninitialized prohibited fields. Even though it
is not used because it is "prohibited", it is simpler to just
initialized it with the value received from a server.

Fixes #81939
Coverity-CID: 434649

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>