slimbus: fix kerneldoc comments

The kerneldoc comments in drivers/slimbus/stream.c were not properly
formatted, leading to a distinctly unsatisfying "no structured comments
found" warning in the docs build.  Sprinkle some asterisks around so that
the comments will be properly recognized.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>