lan9118: log and ignore access to invalid registers, rather than aborting

With this change, access to invalid/unimplemented device registers are
logged as a "guest error" rather than aborting qemu with
hw_error. This enables drivers for similar devices (e.g. SMSC 9221),
by simply ignoring the unimplemented writes. It's also closer to what
real hardware does.

Signed-off-by: Andrew Baumann <Andrew.Baumann@microsoft.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
