staging: comedi: 8255: rename cb_arg and cb_func

The 8255's private data variables cb_arg and cb_func are actually
the iobase for the 8255 device and the pointer to the io function
to access the device. Rename them as such to improve the readability
of the driver.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>