hack: do not use BCH RBs for DL scheduling in subframe 0

The problem is that when using those RBs with MCS 28 we may exceed the code rate 0.93 and according to 36.213 7.1.7 the UE may skip decoding PDSCH entirely in this case.

This is a hack. The real solution is to check that the code rate is below 0.93 for each scheduling decision and, I don't know, reduce the MCS if the code rate is above, so that in the end it is below. That means that we need a proper resource grid for the configuration of the eNB and this is not an easy thing (at least from my point of view) given all the possible configurations for the eNB, so I prefer not to do it rather than do something incorrect, thus this hack.

The problem of this hack is that we won't use all the available RBs for scheduling, potentially reducing the maximum throughput achievable.

To be fixed properly at some time, by someone who understands fully the resource grid and all the possible combinations (fdd/tdd, number of antennas, whatever else).