staging: comedi: addi_apci_3xxx: rename 'dw_AiBase' in private data

This variable holds the ioremap'ed PCI bar 3 used to read/write the
analog input and output registers. Rename it to simply 'mmio'.

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>