NET: sa11x0-ir: split si->dev for IrDA transmit and receive buffers

The sa11x0-ir device is not the device which is doing the DMA, the
DMA is being performed by a separate DMA engine.  Split the struct
device associated with each DMA channel from the main struct device,
but for the time being initialize it from the main struct device.

This is another preparatory step to converting this driver to use the
DMA engine API.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>