thunderbolt: stream: Add support for busy polling

Using interrupts and scheduling workers increase latency so latency
critical applications may want to avoid that. Make this possible in
USB4STREAM by adding a new ConfigFS attribute: busy_poll that, when
activated switches the rings to polling mode. The cost for lower latency
is that this burns more CPU cycles and things like poll(2) cannot be
used.

Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>