dsa: rtl8366: Remove unused variable.

drivers/net/dsa/rtl8366.c: In function ‘rtl8366_reset_vlan’:
drivers/net/dsa/rtl8366.c:234:25: warning: unused variable ‘vlan4k’ [-Wunused-variable]

Signed-off-by: David S. Miller <davem@davemloft.net>