drm/xe/guc_submit: fix xa_store() error checking

drm/xe/guc_submit: fix xa_store() error checking

Looks like we are meant to use xa_err() to extract the error encoded in
the ptr.

Fixes: dd08ebf6c352 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Badal Nilawar <badal.nilawar@intel.com>
Cc: <stable@vger.kernel.org> # v6.8+
Reviewed-by: Badal Nilawar <badal.nilawar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241001084346.98516-7-matthew.auld@intel.com