selftests: hsr: Use `let' properly.

The timeout in the while loop is never subtracted due wrong usage of
`let' leading to an endless loop if the former condition never gets
true.

Put the statement for let in quotes so it is parsed as a single
statement.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/hsr/hsr_ping.sh

@@ -197,7 +197,7 @@ do
 		break
 	fi
 	sleep 1
-	let WAIT = WAIT - 1
+	let "WAIT = WAIT - 1"
 done
 
 # Just a safety delay in case the above check didn't handle it.