workqueue: fix texinfodocs warning for WQ_* flags reference

Sphinx emitted a warning during make texinfodocs:

  WARNING: Inline literal start-string without end-string.

This was caused by the trailing '*' in "%WQ_*" being parsed as
reStructuredText markup in the kernel-doc comment.

Escape the '*' in the comment so that Sphinx treats it as a literal
character, resolving the warning.

Signed-off-by: Kriish Sharma <kriish.sharma2006@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>