soc/google/cpif: Fix OOB read from legacy ipc channel

There are two instances where a pointer from CP shared memory is
used to read from the CP buffer. Without checking for bounds, we risk
OOB read from AP.

Bug: 321714444
Change-Id: I9af053638a85584a9f259c7e43edbf66ee7568ca
Signed-off-by: Mahesh Kallelil <kallelil@google.com>