sandbox: cros_ec: fix uninitialized use of len

sandbox: cros_ec: fix uninitialized use of len

Building with gcc-5.2 raises this warning:

drivers/misc/cros_ec_sandbox.c: In function cros_ec_sandbox_packet:
drivers/misc/cros_ec_sandbox.c:483:5: warning: len may be used uninitialized in this function [-Wmaybe-uninitialized]
  if (len < 0)
     ^

If the function process_cmd() is called with
req_hdr->command == EC_CMD_ENTERING_MODE, the value of len will be
returned uninitialized.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>