net: fec_mxc: Declare 'promisc' as bool

priv->promisc is used as the parameter of the set_promisc() call
which accepts a bool type instead of char.

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>