selftests/powerpc: Lower run time of count_stcx_fail test

The count_stcx_fail test runs for close to or just over 2 minutes, which
means it sometimes times out.

That's overkill for a test that just demonstrates some PMU counters
are working. Drop the 64 billion instruction case, to lower the runtime
to ~30s.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/20241106130453.1741013-1-mpe@ellerman.id.au