V4L/DVB (7699): pvrusb2: Implement statistics for USB I/O performance / tracking

Implement a mechanism in the pvrusb2 driver for gathering statistics
on the stream buffering, including bytes transferred, buffers handled,
buffers in flight, etc.  This is useful for debugging certain classes
of streaming issues and for determining if the buffer pool size is
generally correct for the driver.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>