powerpc: Add missing calls to va_end()

cppcheck picked up that there were a couple of missing va_end()
calls in functions using va_start().

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/prom_init.c

@@ -389,6 +389,7 @@ static void __init prom_printf(const char *format, ...)
 			break;
 		}
 	}
+	va_end(args);
 }