powerpc: Fix build on some non-freescale platforms

Commit 9deaa53a broke build
on platforms that use legacy_serial.c without also having
CONFIG_SERIAL_8250_FSL enabled due to an unconditional code
to a routine in that module.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/legacy_serial.c

@@ -442,8 +442,10 @@ static void __init fixup_port_irq(int index,
 
 	port->irq = virq;
 
+#ifdef CONFIG_SERIAL_8250_FSL
 	if (of_device_is_compatible(np, "fsl,ns16550"))
 		port->handle_irq = fsl8250_handle_irq;
+#endif
 }
 
 static void __init fixup_port_pio(int index,