e1000: drop check_rxov, always treat RX ring with RDH == RDT as empty

Real HW always treats RX ring with RDH == RDT as empty.
Emulation is supposed to behave the same.

Reported-by: Chris Webb <chris.webb@elastichosts.com>
Reported-by: Richard Davies <richard.davies@elastichosts.com>
Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
