staging: comedi: jr3_pci: rename channel to sensor

The driver overloads the term "channel" a lot.  To help reduce
confusion, rename the `channel` member of `struct
jr3_pci_subdev_private` to `sensor` as it points to a `struct
jr3_sensor`.  Also rename the various function parameters and local
variables called `channel` that point to a `struct jr3_sensor` to
`sensor`.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>