BTS: fix as_l1_count_paging(): do not count PCH filling messages

Since build #842, both TC_paging_{i|t}msi_80percent started to fail:

  "BTS_Tests.ttcn:3051 : Expected 271 pagings but have 284"
      BTS_Tests.ttcn:6414 BTS_Tests control part
      BTS_Tests.ttcn:3051 TC_paging_imsi_80percent testcase

  "BTS_Tests.ttcn:3075 : Expected 543 pagings but have 553"
      BTS_Tests.ttcn:6415 BTS_Tests control part
      BTS_Tests.ttcn:3075 TC_paging_tmsi_80percent testcase

As it turns out, since If5339c7a91b4e0188194f1cd935798f153974e01
TITAN can decode PCH filling messages with no Mobile Identity.
We should not count them in as_l1_count_paging(), as they're
sent by osmo-bts itself.

Change-Id: I420f36ed000b1c2488fbe500c33a8161e27d20e3
Fixes: OS#4475
1 file changed