qed_chain_get_element_left{,_u32} returned 0 when the difference
between producer and consumer page count was equal to the total
page count.
Fix this by conditional expanding of producer value (vs
unconditional). This allowed to eliminate normalizaton against
total page count, which was the cause of this bug.

Misc: replace open-coded constants with common defines.

Fixes: a91eb52a ("qed: Revisit chain implementation")
Signed-off-by: NAlexander Lobakin <alobakin@marvell.com>
Signed-off-by: NIgor Russkikh <irusskikh@marvell.com>
Signed-off-by: NMichal Kalderon <michal.kalderon@marvell.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>