net: dsa: cleanup resources upon module removal

Make sure that we unassign the master_netdev dsa_ptr to make the packet
processing go through the regular Ethernet receive path.

Suggested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>