net: ipa: kill ipa_interrupt_add()

The dynamic assignment of IPA interrupt handlers isn't needed; we
only handle three IPA interrupt types, and their handler functions
are now assigned directly.  We can get rid of ipa_interrupt_add()
and ipa_interrupt_remove() now, because they serve no purpose.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>