ne2000: Drop ne2000_can_receive

ne2000_receive already checks the same conditions and drops the packet
if it's not ready, removing the .can_receive callback avoids the
necessity to add explicit flushes when the conditions turn true (which
is required by the new semantics of .can_receive since 6e99c63
"net/socket: Drop net_socket_can_send").

Plus the "return 1" if E8390_STOP is also suspicious.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
