Staging: rtl8192u: Use __packed instead of __attribute__((packed))

This patch fixed the following checkpatch.pl warning:
"WARNING: __packed is preferred over __attribute__((packed))".

Signed-off-by: Cristina Opriceana <cristina.opriceana@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>