dm9601: support dm9620 variant

dm9620 is a newer variant of dm9601 with more features (usb 2.0, checksum
offload, ..), but it can also be put in a dm9601 compatible mode, allowing
us to reuse the existing driver.

This does mean that the extended features like checksum offload cannot be
used, but that's hardly critical on a 100mbps interface.

Thanks to Sławek Wernikowski <slawek@wernikowski.net> for providing me
with a dm9620 based device to test.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>