MLBEDSW-6407: Vela fails with TypeError in npu_performance

 - This is due to calling range() on a non-integer value which in turn is due
to a change in the behaviour of round() on numpy.float64 values
 - The fix is to always force the output of the round() to be an integer and
thereby stop whole number floating point values propagating into the kernel
dimensions which later feed into the range().

Signed-off-by: Tim Hall <tim.hall@arm.com>
Change-Id: Ic75cb6ba85a90c81c1d762067d89a10caaa13b92