net: dsa: mv88e6xxx: use setup_timer to simplify the code

Use setup_timer function instead of initializing timer with the function
and data fields.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>