enhance rate conversion function

luci/luci-app-qmodem/root/usr/share/qmodem/generic.sh

@@ -350,13 +350,24 @@ get_reboot_caps()
 
 rate_convert()
 {
+    #check if bc is installed
+    is_bc_installed=$(which bc)
     local rate=$1
     rate_units="bps Kbps Mbps Gbps"
-    for i in $(seq 0 3); do
-        if [ $rate -lt 1024 ]; then
-            break
-        fi
-        rate=$(($rate / 1024))
-    done
+    if [ -z "$is_bc_installed" ]; then
+        for i in $(seq 0 3); do
+            if [ $rate -lt 1024 ]; then
+                break
+            fi
+            rate=$(($rate / 1024))
+        done
+    else
+        for i in $(seq 0 3); do
+            if [ $(echo "$rate < 1024" | bc) -eq 1 ]; then
+                break
+            fi
+            rate=$(echo "scale=2; $rate / 1024" | bc)
+        done
+    fi
     echo "$rate `echo $rate_units | cut -d ' ' -f $(($i+1))`"
 }