net: fs_enet: merge NAPI RX and NAPI TX

Initially, a NAPI TX routine has been implemented separately from
NAPI RX, as done on the freescale/gianfar driver.

By merging NAPI RX and NAPI TX, we reduce the amount of TX completion
interrupts.

Handling of the budget in association with TX interrupts is based on
indications provided at https://wiki.linuxfoundation.org/networking/napi


We never proceed more than the complete TX ring on a single run.

At the same time, we fix an issue in the handling of fep->tx_free:

It is only when fep->tx_free goes up to MAX_SKB_FRAGS that
we need to wake up the queue. There is no need to call
netif_wake_queue() at every packet successfully transmitted.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>