sh_eth: fix DMA channel misreporting

Currently 'ifconfig' for the Ethernet devices handled by this driver  shows
"DMA chan: ff" while the driver doesn't use any DMA channels. Not assigning
a value to 'net_device::dma' causes 'ifconfig'  to  correctly not report  a
DMA channel.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>