powerpc/powernv: Fix compile without CONFIG_TRACEPOINTS

The functions returns s64 but the return statement is missing.
This adds the missing return statement.

Fixes: 75d9fc7f

 ("powerpc/powernv: move OPAL call wrapper tracing and interrupt handling to C")
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>