net: ipa: use WARN_ON() rather than assertions

I've added commented assertions to record certain properties that
can be assumed to hold in certain places in the IPA code.  Convert
these into real WARN_ON() calls so the assertions are actually
checked, using the standard WARN_ON() mechanism.

Where errors can be returned, return an error if a warning is
triggered.

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