nixos/displaylink: fix with strict shellcheck

using the displaylink module currently fails with
`systemd.enableStrictShellChecks = true;`. there are two warnings:

- `read` without `-r` can technically mangle backslashes, as it acts as
  an escape character by default. as far as we can tell, this shouldn't
  cause problems, but it seems like a harmless and technically-correct
  fix.
- `SUSPEND_RESULT` is unused. as far as i can tell, this indeed seems to
  be the case, so this makes sense to remove.

nixos/modules/hardware/video/displaylink.nix

@@ -46,7 +46,7 @@ in
 
     powerManagement.powerDownCommands = ''
       #flush any bytes in pipe
-      while read -n 1 -t 1 SUSPEND_RESULT < /tmp/PmMessagesPort_out; do : ; done;
+      while read -r -n 1 -t 1 < /tmp/PmMessagesPort_out; do : ; done;
 
       #suspend DisplayLinkManager
       echo "S" > /tmp/PmMessagesPort_in
@@ -54,7 +54,7 @@ in
       #wait until suspend of DisplayLinkManager finish
       if [ -f /tmp/PmMessagesPort_out ]; then
         #wait until suspend of DisplayLinkManager finish
-        read -n 1 -t 10 SUSPEND_RESULT < /tmp/PmMessagesPort_out
+        read -r -n 1 -t 10 < /tmp/PmMessagesPort_out
       fi
     '';