early-IA: use the correct TRX

In early-Immediate-Assignment, the BSC sends the IMM ASS message
directly after it sent the Channel Activation message, and osmo-bts
should cache it until the Channel Activation is complete.

So far the code had a bug: it assumed that the lchan was on the same TRX
where the IMM ASS is transmitted -- but actually, 'trx' refers to the
BCCH channel's TRX, i.e. always c0.

Instead, look up the correct TRX by the ARFCN in the IMM ASS message.

Now, when frequency hopping is enabled, there will be no ARFCN in the
IMM ASS message, hence this fix does not work with frequency hopping.

(To also support frequency hopping, osmo-bsc would need to modify the
RSL protocol: send the IMM ASS message as a custom IE directly as part
of the Channel Activation. Then it is always possible to correllate the
IMM ASS with a specific trx and lchan, no matter what information it
contains. However, early-IA is a "bad" feature in itself as it
"promotes" having high latency on Abis. It seems unnecessary to do extra
work to also support this odd use case for frequency hopping.)

Related: SYS#6655
Change-Id: Id9a930e5c67122812b229dc27ea2bfe246b67611
---
M src/common/rsl.c
1 file changed, 65 insertions(+), 9 deletions(-)