pru UartIO: small fix to write(discard=True)
pru UartIO: small fix to write(discard=True)

write() with discard=True but flush=False should only discard when
polling.  if you want to discard after write, do so manually.

mvduin/py-uio,mvduin/py-uio,mvduin/py-uio,mvduin/py-uio,mvduin/py-uio