caif_virtio: Use vringh_notify_enable correctly

Check on the correct return value from
vringh_notify_enable_kern(). It returns false if
more packets are available, not true.

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
caif_virtio: Check that vringh_config is not null

Check that vringh_config is not NULL before using it.

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>