bts-trx: Avoid submitting first data_ind with FN=0 to upper layers
bts-trx: Avoid submitting first data_ind with FN=0 to upper layers

It can happen that the first burst we receive after enabling the PDCH
channel (when PCU connects to the BTS) is bid!=0. As a result,
chan_state->ul_first_fn is never set and defautl value 0 in there is
passed to the upper layers. As a result, when the 2nd block is
transmitted, this time with correct FN, the PCU will see a huge jump in
FNs. Since in PDCH the bursts are always consecutive, let's simply use
bi->fn - 3 as a first_fn and be done with the issue.

Related: OS#5020
Change-Id: Ie982caeb29f3ffd880b44e88a89b85ea3e6e6947

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