staging: rtl8192u: fix comments

This patch fixes errors raised by checkpatch.pl relating to use of C99 comments
in r8192U_dm.c, and cleans up existing ANSI C comments.
Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>