Initialize wireless interface attributes in proper function

Currently multicast to unicast feature may be configured for incorrect wireless interface in case of reconfiguration.

Test case:

Initial wireless configuration:

config wifi-iface
  option mode ap
  option disabled 1

config wifi-iface
  option mode sta
  option disabled 0

config wifi-iface
  option mode ap
  option disabled 0

After reboot, multicast to unicast feature is configured for interface #3 (wlan0-1) only.

Next, enable interface #1 and issue "wifi" command. Now, multicast to unicast feature is configured for interface #2 (wlan0) which is wrong.
It should be configured for interfaces #1 and #3 only. This patch resolves this problem.

Signed-off-by: Dmitry Ivanov <dima@ubnt.com>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>