selftests/powerpc: Reduce sigfuz runtime to ~60s

The defaults for the sigfuz test is to run for 4000 iterations, but
that can take quite a while and the test harness may kill the test.
Reduce the number of iterations to 600, which gives a runtime of
roughly 1 minute on a Power8 system.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191013234643.3430-1-mpe@ellerman.id.au