common/l1sap.c: limit the minimal ToA for RACH bursts
common/l1sap.c: limit the minimal ToA for RACH bursts

In general, RACH bursts should not arrive with negative offset.
Let's limit early signal arrival up to 2 symbols, otherwise it
is most likely noise, interference or a ghost.

TTCN-3 test case: Icccc88545ed3aabd6da28a40599a8a77d1de477d
Change-Id: I662294fe3136cf7a259be13816a3e63f7db9a948

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