net: dsa: mv88e6xxx: Default CMODE to 1000BaseX only on 6390X

Commit 787799a9 sets the SERDES interfaces of 6390 and 6390X to
1000BaseX, but this is only needed on 6390X, since there are SERDES
interfaces which can be used on lower ports on 6390.

This commit fixes this by returning to previous behaviour on 6390.
(Previous behaviour means that CMODE is not set at all if requested mode
is NA).

This is needed on Turris MOX, where the 88e6190 is connected to CPU in
2500BaseX mode.

Fixes: 787799a9 ("net: dsa: mv88e6xxx: Default ports 9/10 6390X CMODE to 1000BaseX")
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>