net: dsa: use temporary dsa_device_ops variable

When resolving the DSA tagging protocol used by a CPU switch, use a
temporary "tag_ops" variable to store the dsa_device_ops instead of
using directly dst->tag_ops. This will make the future patches moving
this pointer around easier to read.

There is no functional changes.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>