diag: Prevent possible integer overflow while processing cntl pkt

Possible integer overflow while processing control packets received
from peripherals prevented by typecasting the lengths during
buffer boundary check.

Change-Id: Ic29553a8c3422c9e11051d78a6b57a4f921586b9
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>