led-set-all-groups-asserted: fix shellcheck issues

Fix various issues reported by shellcheck:
    - Negative numbers are not valid shell return codes.
    - Numorous variables needing quotes to prevent wordsplits.
    - Prefer (( expr )) over let syntax.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I8f23138e969a49ee4951aedee5fda22cb19ce217
diff --git a/scripts/led-set-all-groups-asserted.sh b/scripts/led-set-all-groups-asserted.sh
index 1375b11..4e8bf91 100755
--- a/scripts/led-set-all-groups-asserted.sh
+++ b/scripts/led-set-all-groups-asserted.sh
@@ -17,17 +17,17 @@
 if [ $# -lt 1 ]; then
     echo "At least ONE argument needed";
     usage;
-    exit -1;
+    exit 1;
 fi
 
 # User passed in argument [true/false]
 action=$1
 
 # If it is not "true" or "false", exit
-if [ $action != "true" ] && [ $action != "false" ]; then
+if [ "$action" != "true" ] && [ "$action" != "false" ]; then
     echo "Bad argument $action passed";
     usage;
-    exit -1;
+    exit 1;
 fi
 
 # Get the excluded groups, where $@ is all the agruments passed
@@ -36,7 +36,7 @@
 
 for arg in "$@"
 do
-   if [ $arg == $action ]
+   if [ "$arg" == "$action" ]
    then
        # Must be true/false
        continue
@@ -46,7 +46,7 @@
    else
        excluded_groups="${excluded_groups}$arg|"
    fi
-   let "index+=1"
+   ((index+=1))
 done
 
 # Now, set the LED group to what has been requested
@@ -54,12 +54,12 @@
 then
     for line in $(busctl tree xyz.openbmc_project.LED.GroupManager | grep -e groups/ | awk -F 'xyz' '{print "/xyz" $2}');
     do
-        busctl set-property xyz.openbmc_project.LED.GroupManager $line xyz.openbmc_project.Led.Group Asserted b $action;
+        busctl set-property xyz.openbmc_project.LED.GroupManager "$line" xyz.openbmc_project.Led.Group Asserted b "$action";
     done
 else
-    for line in $(busctl tree xyz.openbmc_project.LED.GroupManager | grep -e groups/ | grep -Ev $excluded_groups | awk -F 'xyz' '{print "/xyz" $2}');
+    for line in $(busctl tree xyz.openbmc_project.LED.GroupManager | grep -e groups/ | grep -Ev "$excluded_groups" | awk -F 'xyz' '{print "/xyz" $2}');
     do
-        busctl set-property xyz.openbmc_project.LED.GroupManager $line xyz.openbmc_project.Led.Group Asserted b $action;
+        busctl set-property xyz.openbmc_project.LED.GroupManager "$line" xyz.openbmc_project.Led.Group Asserted b "$action";
     done
 fi