Bluetooth: Controller: Clean up HCI macros

Clean up, document and remove the macros used in hci.c
to access the different parameters and calculate lengths
of events and data structures.

Jira: ZEP-726

Change-Id: I476aa9e061dfe429b5181265167bffa203ed151b
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>