staging: comedi: jr3_pci: rename 'channel' to 'block'

The term "channel" is overloaded in this driver.  Rename the `channel`
member of `struct jr3_t` to `block` to reduce confusion.  `block` is an
array of an anonymous `struct` type, with each element covering the
registers for one subdevice.

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