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>

arch/powerpc/platforms/powernv/opal-call.c

@@ -86,6 +86,7 @@ static s64 __opal_call_trace(s64 a0, s64 a1, s64 a2, s64 a3,
 			     s64 a4, s64 a5, s64 a6, s64 a7,
 			      unsigned long opcode, unsigned long msr)
 {
+	return 0;
 }
 
 #define DO_TRACE false