Bluetooth: Controller: df: Add selectable IQ samples to 8 bit conversion

Nordic Radio Direction Finding Extension provides 12 bits wide IQ
samples. The Bluetooth Core Specification 5.3 Vol 4, Part E sections
7.7.65.21 and 7.7.65.22 limits IQ samples to be 8 bits wide.

There are other way to convert 12 bits IQ samples into Bluetooth Core
specification compliant 8 bits IQ samples than ordinary 4 bits right
shift. If one is sure that samples will never go over 10 bits or 8
bits then, it is allowed to use 2 bits right shift or even 8 least
significant bits of 12 bits IQ samples.

The commit introduces a possibility to choose which approach is used
for IQ samples conversion in HCI layer while Host IQ report events
are created.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>