msm: ipa: Fix to unsigned integer underflow

Added code changes to fix the unsigned integer underflow
leads to accessing unmapped memory.

Change-Id: I8148aebd3597ec6ae8c184199afe816f3d80636e
Acked-by: Ashok Vuyyuru <avuyyuru@qti.qualcomm.com>
Signed-off-by: Mohammed Javid <mjavid@codeaurora.org>