net: ethernet: ti: am65-cpsw: Simplify setting supported interface

Convert the existing IF/ELSE statement based approach of setting the
supported_interfaces member of struct "phylink_config", to SWITCH
statements. This will help scale to newer PHY-MODES as well as newer
compatibles.

Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>