apm821xx: WNDAP6X0: add missed uci-default for compat
This should have been included in the previous patch that
resized the kernel partition to fit bigger kernels.
Fixes: 7a6a3494450c ("apm821xx: WNDAP620 + WNDAP660: reorganize partitions for 5.10")
Signed-off-by: Christian Lamparter <[email protected]>