This fixes warnings when CONFIG_DMA_API_DEBUG=y:

	NULL NULL: DMA-API: device driver tries to free DMA memory it has not allocated [device address=0x000000004781a020] [size=64 bytes]
	net eth0: DMA-API: device driver frees DMA memory with different size [device address=0x000000004781a020] [map size=2048 bytes] [unmap size=64 bytes]

Moreover pass the platform device to dma_{,un}map_single which makes
more sense because the logical network device doesn't know anything
about dma.

Passing the platform device was a suggestion by Lothar Waßmann.
Signed-off-by: NUwe Kleine-König <u.kleine-koenig@pengutronix.de>