Bluetooth: Compress the size of struct hci_ctrl

We can reduce the size of the hci_ctrl struct by converting
'bool req_start' to 'u8 req_flags' and making the two function
pointers a union (since only one is ever set at a time).

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