net: qcom/emac: use correct value for SGMII_LN_UCDR_SO_GAIN_MODE0

The documentation says that SGMII_LN_UCDR_SO_GAIN_MODE0 should be
set to 0, not 6, on the Qualcomm Technologies QDF2432.

Signed-off-by: Timur Tabi <timur@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>