comedi: comedi_8255: Rework subdevice initialization functions

Comedi drivers can initialize an 8255 subdevice in I/O space by calling
`subdev_8255_init()`, or in memory-mapped I/O space by calling
`subdev_8255_mm_init()`, or by supplying a call-back function pointer
and context to either of those functions.  Change it so that a new
function `subdev_8255_cb_init()` shall be called instead when supplying
a callback function and context, and remove the call-back function
parameter from `subdev_8255_init()` and `subdev_8255_mm_init()`.

Also rename `subdev_8255_init()` to `subdev_8255_io_init()`.  The
parameters are changing, so might as well rename it at the same time.

Also rename the `regbase` member of `struct subdev_8255_private` to
`context` since this holds the context for the call-back function call.

Cc: Arnd Bergmann <arnd@kernel.org>
Cc: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20230913170712.111719-7-abbotti@mev.co.uk


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>