mediatek: remove unnecessary execute permission bit

Kernel config does not need to be executable. 644 is enough.

Fixes: 25d9df67 ("mediatek: add v5.4 support")
Signed-off-by: Sungbo Eo <mans0n@gorani.run>
[split by targets]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>