qed: Add aux. function translating sb_id -> igu_sb_id

An additional step for relaxing the IGU order assumption, we now add
an auxiliary function that can be used for finding the HW status block
that's associated with a given MSI-x vector.

Signed-off-by: Yuval Mintz <Yuval.Mintz@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>