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>