net: ipa: rename the NAT enumerated type

Rename the ipa_nat_en enumerated type to be ipa_nat_type, and rename
its symbols accordingly.  Add a comment indicating those values are
also used in the IPA status nat_type field.

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