net: dsa: lantiq_gswip: Exclude RMII from modes that report 1 GbE

Exclude RMII from modes that report 1 GbE support. Reduced MII supports
up to 100 MbE.

Fixes: 14fceff4

 ("net: dsa: Add Lantiq / Intel DSA driver for vrx200")
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20210107195818.3878-1-olek2@wp.pl
Signed-off-by: Jakub Kicinski <kuba@kernel.org>