bluetooth: mesh: brg_cfg: fix restoring bridging table

We store only filled up entries, but want to restore the entire table.
`bt_mesh_setting_set` fails if the restored length didn't match the
provided length.

This commit fixes the restoring of the bridging table by allowing to
restore any size as long as the stored size is less than the allocated
one and it is a multiple of the size of a single entry.

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