dm9601: make it clear that dm9620/dm9621a are also supported

The driver nowadays also support dm9620/dm9621a based USB 2.0 ethernet
adapters, so adjust module/driver description and Kconfig help text to
match.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: David S. Miller <davem@davemloft.net>