Bluetooth: Gracefully response to enabling LE on LE only devices

Currently the enabling of LE on LE only devices causes an error. This
is a bit difference from other commands where trying to set the same
existing settings causes a positive response. Fix this behavior for
this single corner case.

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