Bluetooth: Merge send_acl/cmd back to a single callback

Now that we have the buffer type enum as part of the HCI driver API we
can take advantage of it to pass the buffer type information and not
have to have two separate callbacks.

Change-Id: Ib2ee5b1540e532c9b27903e97660a276c1293fbc
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>