powerpc/perf: Reject EBB events which specify a sample_type

Although we already block EBB events which request sampling using
sample_period, technically it's possible for an event to set sample_type
but not sample_period.

Nothing terrible will happen if an EBB event does specify sample_type,
but it signals a major confusion on the part of userspace, and so we do
them the favor of rejecting it.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>