ar8216: rework/fix AR8337 MAC swap handling

ar8216: rework/fix AR8337 MAC swap handling

In r45970 the MAC swap handling was made opt-in, however some boards
have been forgotten during the conversion. Since the reference design
uses this MAC swapping, and pretty much all known boards using this chip
seem to do so too, enabling the swapping is a more reasonable default
than leaving it disabled.

Change the code to still allow boards to opt-out of this.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
SVN-Revision: 47956