Bluetooth: Set LE Default PHY preferences

If the LE Set Default PHY command is supported, the indicate to the
controller that the host has no preferences for transmitter PHY or
receiver PHY selection.

Issuing this command gives the controller a clear indication that other
PHY can be selected if available.

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