selftest/powerpc: Wrong PMC initialized in pmc56_overflow test

Test uses PMC2 to count the event. But PMC1 is being initialized.
Patch to fix it.

Fixes: 3752e453 ('selftests/powerpc: Add tests of PMU EBBs')
Signed-off-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>