staging: rtl8192u: use __packed instead of __attribute__((packed))

This patch fixes the following checkpatch warning:
WARNING: __packed is preferred over __attribute__((packed))

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>