staging: comedi: gsc_hpdi: rename 'hpdi_iobase' in private data

This member in the private data holds the ioremaped PCI BAR2 address
which is the primary base address for the boards registers.

For aesthetics, rename this member 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>