net: ipa: refactor status buffer parsing

The packet length encoded in an IPA packet status buffer is computed
more than once in ipa_endpoint_status_parse().  It is also checked
again in ipa_endpoint_status_skip(), which that function calls.

Compute the length once, and use that computed value later rather
than recomputing it.  Check for it being zero in the parse function
rather than in ipa_endpoint_status_skip().

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