gprs_rlcmac_sched: fix condition for generating dummy blocks on idle

When a PDCH is idle, then the gaps are filled with dummy blocks. OsmoPCU
supports generating the dummy blocks locally, so that a continous stream of
PDCH blocks is sent to L1. However, some BTS models (the OsmoTRX based models
in particular) are able to generate the idle blocks locally. In this case the
PCU should leave the genration of the dummy blocks to the BTS in order to
save processing time and load on the PCUIF interface.

In gprs_rlcmac_sched we already have a flag to skip idle frames in case we do
not use the so called "direct phy access". A similar mechanism also exists in
pcu_l1_if.cpp in function pcu_rx_rts_req_ptcch().

Unfortunately this check is not implemented correctly. The flag gets set when
the ENABLE_DIRECT_PHY define constant is set. However, this does not say
anything about whether the BTS model supports the generation of idle blocks or
not. The define constant is intended to be used to disable direct phy related
code in on platforms where no direct phy code is used or cannot be used. We
must instead check the BTS model (bts->bts_model) in order to decide whether
this particular BTS type requires the generation of dummy blocks or not.

Related: OS#6191
Change-Id: I7a08d8cc670fa14f7206ffffdbc22351f3668a17
---
M src/bts.h
M src/gprs_rlcmac_sched.cpp
M src/pcu_l1_if.cpp
3 files changed, 63 insertions(+), 9 deletions(-)