ipq806x: dwmac: clear forced speed during probe

ipq806x: dwmac: clear forced speed during probe

On a Ubiquiti UniFi AC HD (ubnt,unifi-ac-hd, UAP-AC-HD, UAP301), a
forced speed on gmac1 is set in the QSGMII PCS_ALL_CH_CTL register,
presumably by the bootloader (4.3.28), preventing the interface from
being usable. The QSDK NSS GMAC driver takes care to clear the forced
speed in nss_gmac_qsgmii_dev_init
(https://source.codeaurora.org/quic/qsdk/oss/lklm/nss-gmac/tree/ipq806x/nss_gmac_init.c?h=nss
at d5bb14925861).

gmac1 is connected to the port on the device labeled SECONDARY, and is
currently eth0 but will be switched to eth1 by a subsequent patch. By
clearing the QSGMII PCS forced speed during dwmac initialization when
SGMII is in use, this port becomes usable.

This patch is upstreamable, and will be sent upstream after successful
testing in OpenWrt.

Signed-off-by: Mark Mentovai <mark@moxienet.com>
Build-tested: ipq806x/ubnt,unifi-ac-hd
Run-tested: ipq806x/ubnt,unifi-ac-hd