hinic: Avoid some over memory allocation

'prod_idx' (atomic_t) is larger than 'shadow_idx' (u16), so some memory is
over-allocated.

Fixes: b15a9f37

 ("net-next/hinic: Add wq")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>