net: ipa: add driver shutdown callback

net: ipa: add driver shutdown callback

A system shutdown can happen at essentially any time, and it's
possible that the IPA driver is busy when a shutdown is underway.
IPA hardware accesses IMEM and SMEM memory regions using an IOMMU,
and at some point during shutdown, needed I/O mappings could become
invalid.  This could be disastrous for any "in flight" IPA activity.

Avoid this by defining a new driver shutdown callback that stops all
IPA activity and cleanly shuts down the driver.  It merely calls the
driver's existing remove callback, reporting the error if it returns
one.

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