staging: comedi: drivers: introduce comedi_legacy_detach()

This function is intended to be used by the comedi legacy (ISA) drivers
either directly as the (*detach) function or as a helper in the drivers
private (*detach) function.

Modify the comedi_request_region() helper so that it stores the 'len' of
the region as well as the 'start' after the region has been successfuly
allocated by request_region() in __comedi_request_region(). This region
will then be automatically released detach of the driver by the
comedi_legacy_detach() helper.

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