Receiver HDL works in concert with driver

* Receive flow control.
* Test that receiver can stream 48KSPS.
* rx_crank kicks off the receive DMA pingpong mode.
* Re-ordered read() driver call to handle errors better.
* Made ioctl() MMAP_READ call work smoothly with non-blocking files.
* Simplified the receive fsync() driver call to finish faster.
* Fix typo when copying DMA config from transmit to receive.
* Don't reset mock buffer when clear'ed; fixes the tests.
* Delay a few usecs after clearing DUC/DDC.