Bluetooth: Split HCI command & event buffers to two pools

The HCI commands and events have slightly different requirements
(e.g. max sizes, or the fact that events don't need any user data) so
it makes sense to keep them separate. The total count is kept at 8 (2
+ 6) but the ACL_OUT count is modified to not exceed the event count
(to avoid inability of handling bursts of Number of Completed Packets
events). The ACL_IN count is also modified to keep these symmetric for
now.

Change-Id: Ia1915a596424425525b6df67e0ddce47a7f618f3
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>