Staging: comedi: serial2002: Fix memory leak on detach

The comedi device 'detach' method for the serial2002 driver has an
off-by-one error in its loop for freeing data belonging to its
subdevices.  Fix it.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>