net: arc_emac: prevent reuse of unreclaimed tx descriptors

This patch changes the logic in tx path to ensure that tx descriptors
are reused for transmission only after they have been reclaimed by
arc_emac_tx_clean().

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>