net: mscc: ocelot: disable flow control on NPI interface

The Ocelot switches do not support flow control on Ethernet interfaces
where a DSA tag must be added. If pause frames are enabled, they will be
encapsulated in the DSA tag just like regular frames, and the DSA master
will not recognize them.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>