drm/xe/guc: Return CTB response length

Not all CTB responses from the GuC are fixed size and we need to
pass response length to the caller, if there was a response_buffer.
Easiest solution is to return it as positive value from all
xe_guc_ct_send_recv() functions.  The CTB response length is always
between 1 and 254 (ie. GUC_HXG_MSG_MIN_LEN and GUC_CTB_MAX_DWORDS
- GUC_HXG_MSG_MIN_LEN).

Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20240111152724.497-1-michal.wajdeczko@intel.com


Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>