microchipsw: lan969x: make kernel config version specific

As preparation for enabling 6.18 kernel, rename the existing config to a
version specific 6.12 config.

Signed-off-by: Robert Marko <robert.marko@sartura.hr>

diff --git a/target/linux/microchipsw/lan969x/config-default b/target/linux/microchipsw/lan969x/config-6.12
similarity index 100%
rename from target/linux/microchipsw/lan969x/config-default
rename to target/linux/microchipsw/lan969x/config-6.12