USB: usb-skeleton.c: fix open/disconnect race

If usb device is disconnected between usb_get_intfdata()
and kref_get() in skel_open(), kref_get may access a freed
object.

Also check if device is disconnected in ->open.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>