Both the transmit and receive DMA store identical data: the skb, dma
address, and the dma registers.  Move this data into its own data
structure.  The following replacements were used:

	rxskb -> dma_rx.skb
	rxbuf_dma -> dma_rx.dma
	rxdma -> dma_rx.regs

	txskb -> dma_tx.skb
	txbuf_dma -> dma_tx.dma
	txdma -> dma_tx.regs
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>