irqstats: fix iqrstats on Raspberry Pi (partial revert for 5190244c)

Raspberry Pi has line "FIQ:              usb_fiq" in /proc/interrupts.
"VALUE=$((VALUE + VAL))" results "Illegal number" in dash (Raspberry Pi)
"VALUE=$((VALUE + $VAL))" works in dash, bash and busybox (OpenWrt).

plugins/irqstats

@@ -29,7 +29,7 @@ fetch_irqstats() {
     VALUE=0
     for VAL in $VALS;
     do
-      VALUE=$((VALUE + VAL))
+      VALUE=$((VALUE + $VAL))
     done
     echo "i$ID.value $VALUE"
   done