drm/xe/guc: Fix wrong assert about full_len

This variable holds full length of the message, including header
length so it should be checked against GUC_CTB_MSG_MAX_LEN.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>