net: dsa: lan9303: Move tag setup to new lan9303_setup_tagging

Prepare for next patch:
Move tag setup from lan9303_separate_ports() to new function
lan9303_setup_tagging()

Signed-off-by: Egil Hjelmeland <privat@egil-hjelmeland.no>
Signed-off-by: David S. Miller <davem@davemloft.net>