staging: comedi: addi_apci_1564: introduce apci1564_private struct

The addi_private struct defined in addi-data/addi_common.h is very bloated
and contains many fields which addi_apci_1564 does not require.  In the
interest of eventually removing this driver's dependency on
addi_common.h, we can create a private data struct specifically for
addi_apci_1564 containing only the fields it will actually use.

Signed-off-by: Chase Southwood <chase.southwood@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Cc: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>