net: dsa: b53: Do not clear existing mirrored port mask

Clearing the existing bitmask of mirrored ports essentially prevents us
from capturing more than one port at any given time. This is clearly
wrong, do not clear the bitmask prior to setting up the new port.

Reported-by: Hubert Feurstein <h.feurstein@gmail.com>
Fixes: ed3af5fd ("net: dsa: b53: Add support for port mirroring")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>