net: stmmac: enable tx queue 0 for gmac4 IPs synthesized with multiple TX queues

The dwmac4 IP can synthesized with 1-8 number of tx queues.
On an IP synthesized with DWC_EQOS_NUM_TXQ > 1, all txqueues are disabled
by default. For these IPs, the bitfield TXQEN is R/W.

Always enable tx queue 0. The write will have no effect on IPs synthesized
with DWC_EQOS_NUM_TXQ == 1.

The driver does still not utilize more than one tx queue in the IP.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>