staging: comedi: serial2002: don't assume the number of subdevices to detach

Use the number of subdevices allocated (dev->n_subdevices) in the
(*detach) instead of assuming a given number.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>