caif-hsi: Bugfix - Piggyback'ed embedded CAIF frame lost

When receiving a piggyback'ed descriptor containing an
embedded frame, but no payload, the embedded frame was
lost.

Signed-off-by: Per Ellefsen <per.ellefsen@stericsson.com>
Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>