net: ipa: make IPA interrupt handler threaded only

When the IPA interrupt handler runs, the IPA core clock must already
be operational, and the interconnect providing access by the AP to
IPA config space must be enabled too.

Currently we ensure this by taking a top-level "stay awake" IPA
clock reference, but that will soon go away.  In preparation for
that, move all handling for the IPA IRQ into the thread function.

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