octeontx2-af: fix error code in is_valid_offset()

The is_valid_offset() function returns success/true if the call to
validate_and_get_cpt_blkaddr() fails.

Fixes: ecad2ce8

 ("octeontx2-af: cn10k: Add mailbox to configure reassembly timeout")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YpXDrTPb8qV01JSP@kili
Signed-off-by: Paolo Abeni <pabeni@redhat.com>