staging: comedi: comedi_fops: introduce comedi_dev_from_minor()

A number of functions have to call comedi_get_device_file_info()
to get the comedi_device_file_info pointer for a given minor. That
pointer is only used to get the actual comedi_device pointer for
the minor.

Introduce a new helper function, comedi_dev_from_minor(), to simplify
this operation. 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>