msm: ipa3: Add change to avoid negative index access

During caching of bw reg dereg, there is a chance to access
negative index values of cache. Add changes to overcome it.

Change-Id: I646aa3c35cd5a49e46334a3b09e610448779f72a
Signed-off-by: Praveen Kurapati <pkurapat@codeaurora.org>