drm/xe: tests: fix error message in xe_migrate_sanity_test()

drm/xe: tests: fix error message in xe_migrate_sanity_test()

This is supposed to print the error code but there is a copy and
paste bug so it prints "bo" instead of "err".

Fixes: dd08ebf6c352 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Link: https://patch.msgid.link/an1tu0z3T-qX1ogn@stanley.mountain
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
(cherry picked from commit 28a4198c52a1468fc1b620a9837557ea1dc1766d)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>