lib/oeqa/utils/sshcontrol: correct condition for ending the select() loop

lib/oeqa/utils/sshcontrol: correct condition for ending the select() loop

This was set backwards; per https://docs.python.org/3/library/subprocess.html#subprocess.Popen.returncode
a return code of None indicates the process is still running,
and so the code entered a busyloop that ended on timeout
5 minutes later, lengthening selftests significantly.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>