When a port is hot-unplugged while an app was blocked on a read() call,
the call was unblocked but would not get an error returned.

Return -ENODEV to ensure the app knows the port has gone away.
Signed-off-by: NAmit Shah <amit.shah@redhat.com>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>