sysmobts: fix initial codec mode computation
sysmobts: fix initial codec mode computation

There is no off-by-one between osmocom and L1 definitions...

geosphere/osmo-bts,shimaore/osmo-bts,telenoobie/osmo-bts,tacooper/osmo-bts,osmocom/osmo-bts,telenoobie/osmo-bts,osmocom/osmo-bts,geosphere/osmo-bts,shimaore/osmo-bts,osmocom/osmo-bts,shimaore/osmo-bts,tacooper/osmo-bts,telenoobie/osmo-bts,telenoobie/osmo-bts,shimaore/osmo-bts,geosphere/osmo-bts,geosphere/osmo-bts