Bluetooth: Mesh: Fix enabling scanning after provisioning

There were some needed changes missing after the update to have manual
control of PB-ADV & PB-GATT provisioning bearers. E.g. the test for
CONFIG_BT_MESH_LOW_POWER in net.c was no-longer indicative of scanning
being on or not.

Do the scan enabling selection within the LPN module if LPN is
supported, or otherwise enable scanning unconditionally in
bt_mesh_provision().

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>