Removed the linked list structure, locking, and malloc/free on each USB callback.
Replaced it with a non blocking ring buffer for performance. This resulted
in a large performance improvement when using an Raspberry Pi or Raspberry Pi Zero W
as a rtl_tcp server. The sample rate could be doubled and instead of wired ethernet
wifi could be used. Frequnecy change lag was reduced greatly also.
Signed-off-by: Stephen Blinick <stephen@stesoft.com>
Stephen Blinick