net: systemport: tell RXCHK if we are using Broadcom tags

When Broadcom tags are enabled, e.g: when interfaced to an Ethernet
switch, make sure that we tell the RXCHK engine that it should be
expecting a 4-bytes Broadcom tag after the Ethernet MAC Source Address.

Use netdev_uses_dsa() to check for that condition since that will tell
us if a switch is attached to our network interface.

Fixes: 80105befdb4b ("net: systemport: add Broadcom SYSTEMPORT Ethernet MAC driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>