net: ipa: IPA status preparatory cleanups

The next patch reworks how the IPA packet status structure is
interpreted.  This patch does some preparatory work, to make it
easier to see the effect of that change:
  - Change a few functions that access fields in a IPA packet status
    structure to store field values in local variables with names
    related to the field.
  - Pass a void pointer rather than an (equivalent) status pointer
    to two functions called by ipa_endpoint_status_parse().
  - Use "rule" rather than "val" as the name of a variable that
    holds a routing rule ID.
  - Consistently use "IPA packet status" rather than "status
    element" when referring to this data structure.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>