powerpc: Fix build on some non-freescale platforms

Commit 9deaa53ac7fa373623123aa4f18828dd62292b1a 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>
diff --git a/arch/powerpc/kernel/legacy_serial.c b/arch/powerpc/kernel/legacy_serial.c
index 3fea368..bedd12e 100644
--- a/arch/powerpc/kernel/legacy_serial.c
+++ b/arch/powerpc/kernel/legacy_serial.c
@@ -442,8 +442,10 @@
 
 	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,