pcu: TC_paging_cs_from_sgsn_*: Fix race condition between BSSGP tx and RLCMAC rx

It can happen that the RLCMAC req arrives at osmo-pcu before the
CS_PAGING we send to it over BSSGP, in which case osmo-pcu will schedule
an RLCMAC DL Dummy Block. Take into account this scenario to avoid
failing id it occurs.

Change-Id: I30da93835c7738aefcd84691d83f39759dd4b599
1 file changed