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>