greth: moved TX ring cleaning to NAPI rx poll func

This patch does not affect the 10/100 GRETH MAC.

Before all GBit GRETH TX descriptor ring cleaning was done in
start_xmit(), when descriptor list became full it activated
TX interrupt to start the NAPI rx poll function to do TX ring
cleaning.

With this patch the TX descriptor ring is always cleaned from
the NAPI rx poll function, triggered via TX or RX interrupt.
Otherwise we could end up in TX frames being sent but not
reported to the stack being sent. On the 10/100 GRETH this
is not an issue since the SKB is copied&aligned into private
buffers so that the SKB can be freed directly on start_xmit()

Signed-off-by: Daniel Hellstrom <daniel@gaisler.com>
Signed-off-by: David S. Miller <davem@davemloft.net>