staging: comedi: addi_apci_3120: remove private data 'iobase'

This member of the private data holds the start address of PCI BAR 1 that is
used to access the board registers. The start address is also stored in the
comedi_device 'iobase' member. Use that instead and remove the private data
member.

Remove some unnecessary casts when reading the board registers.

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>