qed: introduce qed_chain_get_elem_used{,u32}()

Add reverse-variants of qed_chain_get_elem_left{,u32}() to be able to
know current chain occupation. They will be used in the upcoming qede
XDP_REDIRECT code.
They share most of the logics with the mentioned ones, so were reused
to collapse the latters.

Signed-off-by: Alexander Lobakin <alobakin@marvell.com>
Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: Michal Kalderon <michal.kalderon@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>