how to use single byte offset in function IORW?

With components other than PIOs, you don't directly control the pins through the IOWR macro. You access registers. All the components have a driver with at least an include file that lists the registers and macros to read/write them.

I don't have any design with a DM9000A but you should check all the include files that are added in your BSP. There must be one of them with a definition of the chip registers.