staging: comedi: jr3_pci: rename struct jr3_channel to jr3_sensor

The driver overloads the term "channel" a lot.  To help reduce
confusion, rename `struct jr3_channel` to `struct jr3_sensor`.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>