staging: comedi: amplc_dio200: add PCI BAR information to board

Add `mainbar` member to `struct dio200_board` to hold the PCI BAR number
for the main registers.  This is `2` for the PCI boards currently
supported (PCI215 and PCI272) but will be different for some new boards
to be supported later.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>