net: fec: increase the size of tx ring and update tx_wake_threshold

When the XDP feature is enabled and with heavy XDP frames to be
transmitted, there is a considerable probability that available
tx BDs are insufficient. This will lead to some XDP frames to be
discarded and the "NOT enough BD for SG!" error log will appear
in the console (as shown below).

[  160.013112] fec 30be0000.ethernet eth0: NOT enough BD for SG!
[  160.023116] fec 30be0000.ethernet eth0: NOT enough BD for SG!
[  160.028926] fec 30be0000.ethernet eth0: NOT enough BD for SG!
[  160.038946] fec 30be0000.ethernet eth0: NOT enough BD for SG!
[  160.044758] fec 30be0000.ethernet eth0: NOT enough BD for SG!

In the case of heavy XDP traffic, sometimes the speed of recycling
tx BDs may be slower than the speed of sending XDP frames. There
may be several specific reasons, such as the interrupt is not
responsed in time, the efficiency of the NAPI callback function is
too low due to all the queues (tx queues and rx queues) share the
same NAPI, and so on.

After trying various methods, I think that increase the size of tx
BD ring is simple and effective. Maybe the best resolution is that
allocate NAPI for each queue to improve the efficiency of the NAPI
callback, but this change is a bit big and I didn't try this method.
Perheps this method will be implemented in a future patch.

This patch also updates the tx_wake_threshold of tx ring which is
related to the size of tx ring in the previous logic. Otherwise,
the tx_wake_threshold will be too high (403 BDs), which is more
likely to impact the slow path in the case of heavy XDP traffic,
because XDP path and slow path share the tx BD rings. According
to Jakub's suggestion, the tx_wake_threshold is at least equal to
tx_stop_threshold + 2 * MAX_SKB_FRAGS, if a queue of hundreds of
entries is overflowing, we should be able to apply a hysteresis
of a few tens of entries.

Fixes: 6d6b39f1 ("net: fec: add initial XDP support")
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>