l1sap: fix handling of lchan->pending_rel_ind_msg
l1sap: fix handling of lchan->pending_rel_ind_msg

After merging the patch [1] fixing handling of the RLL RELease
INDication message in lapdm_rll_tx_cb(), ttcn3-bts-test shows
several regressions:

  pass->fail: BTS_Tests.TC_rll_rel_ind_DCCH_0
  pass->fail: BTS_Tests.TC_rll_rel_ind_DCCH_3
  pass->fail: BTS_Tests.TC_rll_rel_ind_ACCH_0
  pass->fail: BTS_Tests.TC_rll_rel_ind_ACCH_3
  pass->fail: BTS_Tests_LAPDm.TC_sabm_dm

[1] I823c9101bcca72d5792e16379b02d3602ffc2726
    991020c049c63768e147d49bd2918c2d2e0f6dcb

The problem is actually *not* in patch [1], but in the older one
[2] which we attempted to fix.  While a logical channel is in
signalling mode, the lower layers do not produce PRIM_TCH_RTS,
and thus the l1sap_tch_rts_ind() is not being called.

Unlike l1sap_tch_rts_ind(), the l1sap_ph_rts_ind() is being called
regardless of the channel mode (signalling vs speech), so let's
move handling of lchan->pending_rel_ind_msg there.

Change-Id: I2c380f9045624f0a0a8f988bb207bc73d8354857
Fixes: [2] Ie4f70c75f0137b4bd72d579b3a32575bac2fca3

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