dyn PDCH: lc15: add ts_connect_as(), absorbing ts_connect() guts
dyn PDCH: lc15: add ts_connect_as(), absorbing ts_connect() guts

Analogous to 294fbe104b9fb12f06f7cebe68a4b7a6ed28d5ef on osmo-bts-sysmo.

For upcoming dyn PDCH switching, I want to be able to set the pchan dynamically
upon ts_connect() and not continue with OPSTART ACK, but with the dyn PDCH.

Thus recoin ts_connect(ts) to ts_connect_as(ts, pchan, cb) and leave
ts_connect() as a thin wrapper to leave init code unchanged.

Change-Id: I7a27193168f83e8c40b6e54d1842f4502d0475e5

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