net: stmmac: qcom-ethqos: remove MAC_CTRL_REG modification

When operating in "SGMII" mode (Cisco SGMII or 2500BASE-X), qcom-ethqos
modifies the MAC control register in its ethqos_configure_sgmii()
function, which is only called from one path:

stmmac_mac_link_up()
+- reads MAC_CTRL_REG
+- masks out priv->hw->link.speed_mask
+- sets bits according to speed (2500, 1000, 100, 10) from priv->hw.link.speed*
+- ethqos_fix_mac_speed()
| +- qcom_ethqos_set_sgmii_loopback(false)
| +- ethqos_update_link_clk(speed)
| `- ethqos_configure(speed)
| `- ethqos_configure_sgmii(speed)
| +- reads MAC_CTRL_REG,
| +- configures PS/FES bits according to speed
| `- writes MAC_CTRL_REG as the last operation
+- sets duplex bit(s)
+- stmmac_mac_flow_ctrl()
+- writes MAC_CTRL_REG if changed from original read
...

As can be seen, the modification of the control register that
stmmac_mac_link_up() overwrites the changes that ethqos_fix_mac_speed()
does to the register. This makes ethqos_configure_sgmii()'s
modification questionable at best.

Analysing the values written, GMAC4 sets the speed bits as:
speed_mask = GMAC_CONFIG_FES | GMAC_CONFIG_PS
speed2500 = GMAC_CONFIG_FES B14=1 B15=0
speed1000 = 0 B14=0 B15=0
speed100 = GMAC_CONFIG_FES | GMAC_CONFIG_PS B14=1 B15=1
speed10 = GMAC_CONFIG_PS B14=0 B15=1

Whereas ethqos_configure_sgmii():
2500: clears ETHQOS_MAC_CTRL_PORT_SEL B14=X B15=0
1000: clears ETHQOS_MAC_CTRL_PORT_SEL B14=X B15=0
100: sets ETHQOS_MAC_CTRL_PORT_SEL | B14=1 B15=1
ETHQOS_MAC_CTRL_SPEED_MODE
10: sets ETHQOS_MAC_CTRL_PORT_SEL B14=0 B15=1
clears ETHQOS_MAC_CTRL_SPEED_MODE

Thus, they appear to be doing very similar, with the exception of the
FES bit (bit 14) for 1G and 2.5G speeds.

Given that stmmac_mac_link_up() will write the MAC_CTRL_REG after
ethqos_configure_sgmii(), remove the unnecessary update in the
glue driver's ethqos_configure_sgmii() method, simplifying the code.

Konrad states:

Without any additional knowledge, the register description says:

2500: B14=1 B15=0
1000: B14=0 B15=0
100: B14=1 B15=1
10: B14=0 B15=1

Tested-by: Mohd Ayaan Anwar <mohd.anwar@oss.qualcomm.com>
Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1vEPlg-0000000CFHY-282A@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>