net: stmmac: qcom-ethqos: move loopback disable to .mac_finish()

Loopback is enabled to allow the dwmac soft reset to succeed. This
is enabled when clocks are enabled in ethqos_clks_config(), which
happens at driver probe and runtime PM resume - e.g. when the
network device is administratively brought up.

Currently, the loopback is disabled when the link comes up (via
.mac_link_up() calling this driver's .fix_mac_speed().)

Move the qcom_ethqos_set_sgmii_loopback() call which disables
loopback from ethqos_fix_mac_speed() into ethqos' SerDes specific
.mac_finish() method so that loopback is disabled a little earlier
after reset has completed, and dwmac setup has completed.

Reviewed-by: Mohd Ayaan Anwar <mohd.anwar@oss.qualcomm.com>
Tested-by: Mohd Ayaan Anwar <mohd.anwar@oss.qualcomm.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1vuSKq-0000000AScA-1Wh3@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>