The previous stmmac_xdp_set_prog() implementation uses stmmac_release()
and stmmac_open() which tear down the PHY device and causes undesirable
autonegotiation which causes a delay whenever AFXDP ZC is setup.

This patch introduces two new functions that just sufficiently tear
down DMA descriptors, buffer, NAPI process, and IRQs and reestablish
them accordingly in both stmmac_xdp_release() and stammac_xdp_open().

As the results of this enhancement, we get rid of transient state
introduced by the link auto-negotiation:

$ ./xdpsock -i eth0 -t -z

 sock0@eth0:0 txonly xdp-drv
                   pps            pkts           1.00
rx                 0              0
tx                 634444         634560

 sock0@eth0:0 txonly xdp-drv
                   pps            pkts           1.00
rx                 0              0
tx                 632330         1267072

 sock0@eth0:0 txonly xdp-drv
                   pps            pkts           1.00
rx                 0              0
tx                 632438         1899584

 sock0@eth0:0 txonly xdp-drv
                   pps            pkts           1.00
rx                 0              0
tx                 632502         2532160
Reported-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Tested-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>