qed: Fix corner case for chain in-between pages

The amount of chain next pointer elements between the producer
and the consumer indices depends on which pages they currently
point to. The current calculation is based only on their difference,
and it can lead to a number of free elements which is higher by 1
than the actual value.

Signed-off-by: Tomer Tayar <Tomer.Tayar@qlogic.com>
Signed-off-by: Manish Chopra <manish.chopra@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>