mfd: dln2: Add a limit check for invalid echo

The echo field in dln2_transfer_complete comes directly from an USB
transfer and we should not trust it is valid.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>