staging: comedi: cb_pcidas: rename private data 'adc_fifo'

This member of the private data holds the PCI BAR2 resource starting
address. For aesthetics, rename this member to 'pcibar2'.

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>