msm: cpp: Fix for buffer overflow in cpp.

Fix for buffer overflow while handling ioctl.
Instead of checking for length boundary, fix checks
for exact length.

CRs-Fixed: 518731
Change-Id: I9002f84b219e8b06ae0672d87c2d999e728a75aa
Signed-off-by: Krishnankutty Kolathappilly <kkolatha@codeaurora.org>