virtio: mmio: fix signature checking for BE guests

As virtio-mmio config registers are specified to be little-endian,
using readl() to read the magic value and then memcmp() to check it
fails on BE (as readl() has an implicit swab).

Fix it by encoding the magic value as an integer instead of a string.

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>
Acked-by: NPawel Moll <pawel.moll@arm.com>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

drivers/virtio/virtio_mmio.c

@@ -471,7 +471,7 @@ static int virtio_mmio_probe(struct platform_device *pdev)
 
 	/* Check magic value */
 	magic = readl(vm_dev->base + VIRTIO_MMIO_MAGIC_VALUE);
-	if (memcmp(&magic, "virt", 4) != 0) {
+	if (magic != ('v' | 'i' << 8 | 'r' << 16 | 't' << 24)) {
 		dev_warn(&pdev->dev, "Wrong magic value 0x%08lx!\n", magic);
 		return -ENODEV;
 	}