ffac-ssid-changer: replace string indexing

In POSIX sh, string indexing is not defined. This only works in ksh, bash
or busybox's modified version of ash. Does not work in dash, ash, etc.

Fixes:
SC3057 (warning): In POSIX sh, string indexing is undefined.

ffac-ssid-changer/shsrc/ssid-changer.sh

@@ -55,11 +55,11 @@ if [ "$SETTINGS_SUFFIX" = 'nodename' ]; then
 	# 32 would be possible as well
 	if [ ${#SUFFIX} -gt $((30 - ${#PREFIX})) ]; then
 		# calculate the length of the first part of the node identifier in the offline-ssid
-		HALF=$(( (28 - ${#PREFIX} ) / 2 ))
-		# jump to this charakter for the last part of the name
-		SKIP=$(( ${#SUFFIX} - HALF ))
+		max_suffix_length=$(( (28 - ${#PREFIX} ) / 2 ))
 		# use the first and last part of the nodename for nodes with long name
-		SUFFIX=${SUFFIX:0:$HALF}...${SUFFIX:$SKIP:${#SUFFIX}}
+		suffix_first_chars="$(printf '%s' "$SUFFIX" | head -c ${max_suffix_length})"
+		suffix_last_chars="$(printf '%s' "$SUFFIX" | tail -c ${max_suffix_length})"
+		SUFFIX="${suffix_first_chars}...${suffix_last_chars}"
 	fi
 elif [ "$SETTINGS_SUFFIX" = 'mac' ]; then
 	SUFFIX="$(uci -q get network.bat0.macaddr | /bin/sed 's/://g')"