module: preferred way to use MODULE_AUTHOR

For the longest time now we've been using multiple MODULE_AUTHOR()
statements when a module has more than one author, but the comment here
disagrees.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>