xen/netback: fix incorrect usage of RING_HAS_UNCONSUMED_REQUESTS()

Commit 6fac592c ("xen: update ring.h") missed to fix one use case
of RING_HAS_UNCONSUMED_REQUESTS().

Reported-by: Jan Beulich <jbeulich@suse.com>
Fixes: 6fac592c ("xen: update ring.h")
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu@kernel.org>
Link: https://lore.kernel.org/r/20220530113459.20124-1-jgross@suse.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>