net: dsa: Use true instead of 1 in the set_promisc() call

set_promisc() call accepts the parameter of a bool type. Make it
clear by using true instead of 1.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>