msm: IPA: mhi_proxy: correct the return for mhi unvote on ssr

When 4G ssr happens, imp will unregister the mhi. And if the devote
request from Q6 we would need to ignore it. The current handling is
to just ignore this QMI request but if Q6 shutdown is not happen on
time there would be a QMI assert happen on Q6 side. The fix here is
to still response to QMI with error imcompatiable.

Change-Id: I51637181a428a96fcd4385bcb1573acbff808813
Signed-off-by: Bojun Pan <bojunp@codeaurora.org>