Staging: fbtft: Convert comment from C99 style to C89 style

This patch fixes checkpatch.pl error
ERROR: do not use C99 // comments

Signed-off-by: Melike Yurtoglu <aysemelikeyurtoglu@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>