common/l1sap: increase ToA precision for packet Access Bursts
common/l1sap: increase ToA precision for packet Access Bursts

QTA is a Timing Advance value in units of 1/4 of a symbol. Let's
use ToA256 (1/256 of a symbol) field of L1SAP RACH.ind as a base
for QTA calculation in order to achieve better precision.

Change-Id: I6e6fa7985c430a9bdbd12af2a8b2a5a66f11a41c

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