Bluetooth: Mesh: Fix dev key selection in SAR Cfg Client

For dev key encrypiton, BT_MESH_KEY_DEV_REMOTE should be used instead of
BT_MESH_KEY_DEV regardless of whether the destination is a local element
or remote.

BT_MESH_MSG_CTX_INIT_DEV macro selects correct key.

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