net: dsa: bcm_sf2: Move code enabling Broadcom tags

In preparation for enabling Broadcom tags on different ports based on
configuration information, dedicate a function that is responsible for
enabling Broadcom tags for a given port and update the IMP port setup to
call it.

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