net: ethernet: mediatek: remove return value check of `mtk_wed_hw_add_debugfs`

Smatch complains that:
mtk_wed_hw_add_debugfs() warn: 'dir' is an error pointer or valid

Debugfs checks are generally not supposed to be checked
for errors and it is not necessary here.

fix it by just deleting the dead code.

Signed-off-by: Wang Zhang <silver_code@hust.edu.cn>
Reviewed-by: Dongliang Mu <dzm91@hust.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>