net: dsa: unexport dsa_is_port_initialized

The dsa_is_port_initialized helper is only used by dsa_switch_resume and
dsa_switch_suspend, if CONFIG_PM_SLEEP is enabled. Make it static to
dsa.c.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>