wifi-scripts: add missing na_mcast_to_ucast default when proxy_arp is disabled

wifi-scripts: add missing na_mcast_to_ucast default when proxy_arp is disabled

Adds missing default value for na_mcast_to_ucast when proxy_arp is not enabled
in order to silence the following msg:

   daemon.notice: netifd: radio1 (xxxx): sh: out of range

Fixes: bcdb29f78f5 ("wifi-scripts: add na_mcast_to_ucast option")
Reported-by: Antony Kolitsos <zeusomighty@hotmail.com>
Signed-off-by: Rany Hany <rany_hany@riseup.net>
Link: https://github.com/openwrt/openwrt/pull/20806
Signed-off-by: Robert Marko <robimarko@gmail.com>