msm: ipa3: disable prefetch in ipa mhi driver

The prefetch mechanism is not supported yet due to a known limitation
in GSI firmware. This change configures the MHI HW accelerated channels
to work in direct mode rather than doorbell mode.

Change-Id: I8cfc7c941c9c86b810c23935ab8bb8fdc07a41a7
Signed-off-by: Nadine Toledano <nadinet@codeaurora.org>