octphy: Remove code duplication for BER / RSSI conversion
octphy: Remove code duplication for BER / RSSI conversion

Let's introduce some functions to hide the details of BER and RSSI
conversion from OCTPHY representation to L1SAP representation.

Change-Id: I517669c87a97b2ba164a2812811c8802fe0b92e8

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts