brcm47xx: fix switch port mapping on Asus RT-N12 and RT-N16 models

brcm47xx: fix switch port mapping on Asus RT-N12 and RT-N16 models

On Asus RT-N12 and RT-N16 models, the WAN and LAN4 ports are swapped in the
initial switch configuration since the presets present in nvram appear to be
wrong.

Add special casing for these models to detect_by_model() in order to ensure
a proper switch configuration.

Fixes FS#502.

(cherry picked from commit 96ed69101da254b0cb61a0dfc42bd48d27bfacb9
  and squashed with commit f2fdd68664cdf09075e6f18b20946e41a22284b2)

Signed-off-by: Jo-Philipp Wich <jo@mein.io>