net: dsa: mv88e6xxx: fix 6085 frame mode masking

The register bits used for the frame mode were masked with DSA (0x1)
instead of the mask value (0x3) in the 6085 implementation of
port_set_frame_mode. Fix this.

Fixes: 56995cbc

 ("net: dsa: mv88e6xxx: Refactor CPU and DSA port setup")
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>