net: dsa: rename dsa_slave_upper_vlan_check to something more suggestive

We'll be adding a new check in the PRECHANGEUPPER notifier, where we'll
need to check some VLAN uppers. It is hard to do that when there is
already a function named dsa_slave_upper_vlan_check. So rename this one.

Not to mention that this function probably shouldn't have started with
"dsa_slave_" in the first place, since the struct net_device argument
isn't a DSA slave, but an 8021q upper of one.

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>