staging: comedi: amplc_dio200: add min register region size in board

Store the length of the main register region in new member `mainsize` of
`struct dio200_board` and initialize it in each element of
`dio200_boards[]`.  For all currently supported boards this is
initialized to `DIO200_IO_SIZE` (0x20), but will be different for newer
boards to be supported in the future.

For ISA boards, this is the actual region length requested, but for PCI
boards it's the minimum expected region length.  Verify that the PCI BAR
region length is sufficient in `dio200_attach_pci()`.

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