hdl: Adding 32-bit counter mode.

Added a 32-bit counter mode to better understand how many dropped
have occurred during bandwidth testing.
hdl: Updating simulation models and RX FIFO.

Updated the simulation models to be more accurate and run appropriately
with the compile.do script in the platforms modelsim directory.

Also fixed the RX FIFO to clear its contents when we have disabled
reception.

Currently a single DMA transfer worth of samples is still invalid.
hdl: Modified constraints of GPIF drive strength.