msm: gsi: Fix the WARNING for gsi_write_channel_scratch

Definition of the __gsi_update_mhi_channel_scratch is moved to the end
where it is followed by no other non static function.

Change-Id: I6243fbb322c504d606bd5cd02302627afadc82ea
Signed-off-by: Prakruthi Deepak Heragu <pheragu@codeaurora.org>