staging: comedi: use comedi_dev_from_minor()

Remove the need to export comedi_get_device_file_info() by using the
new helper comedi_dev_from_minor(). This will also allow us to make
the comedi_device_file_info struct private.

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>