staging: comedi: s626: remove encpriv macro

The `encpriv` macro relies on a local variable `dev` (of type `struct
comedi_device *`) being set correctly.  By a convoluted path involving
the `private` data pointer of subdevice 5 (the encoder (counter)
subdevice), the macro always yields a pointer to the first element of
the static array `enc_private_data[]`.  That holds statically constant
data for each of 6 encoder channels.

Instead of using the `encpriv` macro, just access the array it points to
directly and get rid of the macro.  Don't bother initializing the
`private` member of the encoder subdevice any more.  Since
`enc_private_data[]` now has nothing to so with subdevice private data,
rename `enc_private_data[]` to `s626_enc_chan_info[]` and rename its
type from `struct enc_private` to `struct s626_enc_info`.  Since the
array contains unchanging, static information, declare it `const` and
declare all the variables that point to it `const`.

A forward declaration of `s626_enc_chan_info[]` has been added
temporarily and will be removed by a later patch.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>