protocol-native: Fix source latency calculation in ADJUST_LATENCY mode

protocol-native: Fix source latency calculation in ADJUST_LATENCY mode

This fixes buffer attr calculation so that we set the source latency to
the requested latency. This makes sense because the intermediate
delay_memblockq is just a mechanism to send data to the client. It
should not actually add to the total latency over what the source
already provides.

With this, the meaning of fragsize and maxlength become more
meaningful/accurate with regards to ADJUST_LATENCY mode -- fragsize
becomes the latency the source is configured for (which is then
approximately the total latency until the buffer reaches the client).
Maxlength, as before, continues to be the maximum amount of data we
might hold for the client before overrunning.