net: stmmac: Fix null-function call in ISR on stmmac1000

(resent due to overhelpful mail client corrupting patch)

At least on Meson GXBB, the CORE_IRQ_MTL_RX_OVERFLOW interrupt is thrown
with the stmmac1000 driver, which does not support set_rx_tail_ptr. With
this patch and the clock fixes, 1G ethernet works on ODROID-C2.

Signed-off-by: Matt Corallo <git@bluematt.me>
Signed-off-by: David S. Miller <davem@davemloft.net>