OML: move BTS number check into separate function
OML: move BTS number check into separate function

* move code which checks for BTS number (obtained via OML) validity into
  separate function
* adjust log messages to match the check
* add spec reference and comments

Change-Id: Id5714fbed910696d30e18d1f20f4c9fced4b9230
Related: OS#2317

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