dl2k: Implement suspend

Add suspend/resume support to dl2k driver.
This requires RX/TX rings to be reset so split out the required
functionality from alloc_list() into new rio_reset_ring().

Tested on Asus NX1101 (IP1000A) and D-Link DGE-550T (DL-2000).

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: David S. Miller <davem@davemloft.net>