cs-etm: avoid casting variable

Because of two's complement reprensentation, casting an int to
and unsigned value doesn't simply get rid of the negative sign.
As such a value of -1 becomes 0xFFFFFFFF, which is clearly not
the desired effect.

This patch deals with cases when @cpu has the value of -1.  In
those cases queue '0' is initially selected.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>