Bluetooth: Add helper macros for accessing Bluetooth user data

To avoid unnecessary stack variables and to simplify code, add helper
macros to access the Bluetooth specific ACL and HCI user data that's
part of the buffers.

Change-Id: Idc337537721115c4091982c04f8a05bbbf0d5a19
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>