virtio-mmio: fix wrong comment about register offset

The register offset of InterruptACK is 0x064, not 0x060.
Fix it.

Signed-off-by: Ryota Ozaki <ozaki.ryota@gmail.com>
Reviewed-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Acked-by: Pawel Moll <pawel.moll@arm.com>
CC: Jiri Kosina <trivial@kernel.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/virtio/virtio_mmio.c

@@ -75,7 +75,7 @@
  *
  * 0x050  W  QueueNotify      Queue notifier
  * 0x060  R  InterruptStatus  Interrupt status register
- * 0x060  W  InterruptACK     Interrupt acknowledge register
+ * 0x064  W  InterruptACK     Interrupt acknowledge register
  * 0x070  RW Status           Device status register
  *
  * 0x100+ RW                  Device-specific configuration space