pata_sil680: Fix build on arch/ppc

Commit 0f436eff breaks build on
arch/ppc as it doesn't implement the machine_is() macro.

This fixes it by using CONFIG_PPC_MERGE instead which represents
arch/powerpc only, while CONFIG_PPC is set for both.
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: NJeff Garzik <jeff@garzik.org>

drivers/ata/pata_sil680.c

@@ -270,7 +270,7 @@ static u8 sil680_init_chip(struct pci_dev *pdev, int *try_mmio)
 		tmpbyte & 1, tmpbyte & 0x30);
 
 	*try_mmio = 0;
-#ifdef CONFIG_PPC
+#ifdef CONFIG_PPC_MERGE
 	if (machine_is(cell))
 		*try_mmio = (tmpbyte & 1) || pci_resource_start(pdev, 5);
 #endif