usbnet: remove ifdefed out call to dma_supported

Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>