net: stmmac: qcom-ethqos: rename "por" members to "rgmii_por"

Rename the "por" and "num_por" members to indicate that they are for
RGMII mode only as ethqos_configure_rgmii() is the only place that the
values are programmed into the registers.

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/E1vuSKV-0000000ASbg-28JK@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>