Ulsch rx 2ant fix

fixes small bug in ulsim which kills one RX antenna in channel simulation. Another simple but bad bug in ulsch_demodulation.c (bad access of avgU array). This probably resulted in a performance degradation for non-ideal channels, even for 1 antenna. The avgU was read in position 1 instead of 0 for 1-antenna and in positions 1 and 2 instead of 0 and 1 for 2-antennas.