msm: ipa: disable prefetch for MHI

MHI might be configured to MBIM aggregation.
For this to work TX prefetch needs to be disabled.

Change-Id: I52510f3a7fbe90d5a472703363136e6e909d301c
Acked-by: Ady Abraham <adya@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>