mac80211: don't call md5sum on non-existing file

If no AP is configured, hostapd-${phy}.conf is not being created,
hence md5sum fails and causes log pollution:

netifd: radio1 (3183): md5sum: can't open '/var/run/hostapd-phy1.conf': No such file or directoy

Hence make sure the file exists when calling md5sum.

Fixes: a5bc9787d4 ("mac80211: add support for dynamically reconfiguring wifi")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>

package/kernel/mac80211/files/lib/netifd/wireless/mac80211.sh

@@ -925,7 +925,7 @@ drv_mac80211_setup() {
 	for_each_interface "sta adhoc mesh monitor" mac80211_prepare_vif
 	NEWAPLIST=
 	for_each_interface "ap" mac80211_prepare_vif
-	NEW_MD5=$(md5sum ${hostapd_conf_file})
+	NEW_MD5=$(test -e "${hostapd_conf_file}" && md5sum ${hostapd_conf_file})
 	OLD_MD5=$(uci -q -P /var/state get wireless._${phy}.md5)
 	if [ "${NEWAPLIST}" != "${OLDAPLIST}" ]; then
 		mac80211_vap_cleanup hostapd "${OLDAPLIST}"