staging: comedi: das08: Add helper functions to check bus type

Add inline helper function `is_isa_board(board)` to check if the driver
supports ISA boards and this is an ISA board, and `is_pci_board(board)`
to check if the driver supports PCI boards and this is a PCI board.

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