net: dsa: lantiq_gswip: fix register definition

I compared the register definitions with the D-Link DWR-966
GPL sources and found that the PUAFD field definition was
incorrect. This definition is unused and causes no issues.

Fixes: 14fceff4 ("net: dsa: Add Lantiq / Intel DSA driver for vrx200")
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>