net/can/af_can.c: Change del_timer to del_timer_sync

This is important for SMP platform to check if timer function is
executing on other CPU with deleting the timer.

Signed-off-by: Rajan Aggarwal &lt;Rajan Aggarwal <rajan.aggarwal85@gmail.com>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: David S. Miller <davem@davemloft.net>