abis: Move FSM registration to constructor function
abis: Move FSM registration to constructor function

We commonly do it this way since there's only need to call it once at
the startup of the program.

Change-Id: I6ee5b89175ee32bc12a8afc9682a8f90d1713291

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