net: dsa: mv88e6xxx: make mv88e6xxx_g1_stats_wait static

mv88e6xxx_g1_stats_wait has no users outside global1.c, so make it
static.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>