staging: comedi: jr3_pci: separate out block type

`struct jr3_t` contains a single array member `block` of a tag-less
`struct` type.  Rename the tag-less `struct` type to `struct jr3_block`
and move its definition outside of `struct jr3_t`.  This will allow us
to use pointers of this type.

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