Bluetooth: Fix sending write_scan_enable when BR/EDR is disabled

We should only send the HCI_Write_Scan_Enable command from
mgmt_set_powered_failed() when BR/EDR support is enabled. This is
particularly important when the discoverable setting is also tied to LE.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>