staging: comedi: drivers: introduce comedi_request_region()

Introduce a helper function to handle the request_region() for legacy
comedi drivers.

As pointed out by Ian Abbott, legacy devices are configured manually
with the "comedi_config" program. The error messages are useful
diagnostics when trying to attach to these boards.

Providing a helper function allows consolidating the error messages
in the drivers and providing a consistent format for the errors.

This helper also sets the dev->iobase automatically for the driver
if the request_region() is successful.

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