osmo-bts-trx: use a lookup table in trx_sched_meas_avg()
osmo-bts-trx: use a lookup table in trx_sched_meas_avg()

This should be better performance-wise and would also allow to avoid
having the same switch statement duplicated in functions added later.

Change-Id: If8124dcd38e7c8408a9f3b9a574d9e3181a2eb15

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