Bluetooth: Controller: Redesign response buffer allocation for commands

With an upcoming patch the HCI command buffers will be reused for the
Command Complete/Status HCI events. The controller HCI code needs some
refactoring to postpone the event buffer allocation so that the
parsing state of the HCI command buffer isn't touched.

Change-Id: I1f614fb9f02ed1886ca84198fbc8c64abc29e44f
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>