staging: comedi: drivers: refactor comedi_request_region()

Split comedi_request_region() into two helper functions.

__comedi_request_region()
Handles the actual request_region() call.

comedi_request_region()
Calls __comedi_request_region() and then sets dev->iobase if the
request was successful.

This allows drivers to use the __comedi_request_region() helper
to handle the request without setting the dev->iobase.

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>