staging: comedi: addi_apci_3120: rename private data 'i_IobaseAddon'

This member of the private data holds the start address of PCI BAR 2 that is
used to access the AMCC Add-On registers.

Rename this CamelCase member and fix its type.

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