Bluetooth: Fix bt_le_scan_update dependency on CONFIG_BLUETOOTH_CENTRAL

This function should depend on BLUETOOTH_CENTRAL and not on
BLUETOOTH_CONN. It should also at least be capable of disabling
scanning even if BLUETOOTH_CENTRAL is not set.

Change-Id: Ic6ccd7ddf07ac6529c64889257dc9f9dfb34b531
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>