bluetooth: mesh: brg_cfg_cli: copy buf in synchronous api

When a synchronous API is used, the content of `buf` will not be
valid by the time the thread that called the synchronous API is woken up
again.

Therefore, the simplest way to solve this is when a user allocates the
buffer which will be filled up with the content of the buffer passed to
the model callback.

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