mhi: Fix compile errors

module_init() was incorrectly used instead of module_exit(). Fix
the macro and the exit function for the correct prototype. Slightly
adjust a function callback to match the header prototype and add a
missing variable.

Fixes: 689af8a2e0a ("mhi: add snapshot for MHI driver stack").

Change-Id: Ic0dedbad55f535efd0d77b9bb3ff0ea1e25d6f3a
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>