Change the return type of ipa_interrupt_config() to be an error
code rather than an IPA interrupt structure pointer, and assign the
the pointer within that function.

Change ipa_interrupt_deconfig() to take the IPA pointer as argument
and have it invalidate the ipa->interrupt pointer.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>