ore: Remove redundant dev_order(), more cleanups

Two cleanups:
* si->cur_comp, si->cur_pg where always calculated after
  the call to ore_calc_stripe_info() with the help of
  _dev_order(...). But these are already calculated by
  ore_calc_stripe_info() and can be just set there.
  (This is left over from the time that si->cur_comp, si->cur_pg
   were only used by raid code, but now the main loop manages
   them anyway even though they are ultimately not used in
   none raid code)

* si->cur_comp - For the very last stripe case, was set inside
  _ore_add_parity_unit(). This is not clear and will be wrong
  for coming raid6 so move this to only caller. Now si->cur_comp
  is only manipulated within _prepare_for_striping(), always next
  to the manipulation of cur_dev.
  Which is much easier to understand and follow.

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>