qed: Distinguish between sb_id and igu_sb_id

In qed code, sb_id means 2 different things:
  - An interrupt vector [usually when received as a parameter from
    a protocol driver, but not only] that's associated with a status
    block.

  - An index to a status block entity existing in HW.

This patch renames the references to the HW entity, adding an 'igu_'
prefix to allow an easier distinction.

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