Bluetooth: Reuse HCI command buffers for the command response

Reduce the pressure on the common RX buffer pool by reusing HCI
command buffers also for the Command Status or Command Complete
response to them. This also implies removing the existing Kconfig
variable for the command buffer sizes since the size is also dependent
on maximum Command Complete event sizes. Instead, reuse the RX buffer
size also for HCI Command buffers.

Change-Id: I006b287d64a0c9ca40de741aa9a424a49a927385
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>