gsi: fix updating mhi channel scratch

Only polling mode is modified dynamically after the channel is
started. Make changes to update the channel scratch only for the
polling bit to avoid oveririding other configuration in MHI v2
mode.

Change-Id: I9aefbf7e32984fce4bfa034a92571336a6721dc4
Signed-off-by: Chaitanya Pratapa <cpratapa@codeaurora.org>