lib/oeqa: sshcontrol: fix false timeout failures

lib/oeqa: sshcontrol: fix false timeout failures

Ocasionally AB shows odd false fails like:
http://autobuilder.yoctoproject.org/main/builders/nightly-arm/builds/1/steps/Running%20Sanity%20Tests/logs/stdio
This should fix that by checking for eof instead of
polling the return code of the ssh process, because the process
might still be there.

Signed-off-by: Stefan Stanacar <stefanx.stanacar@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>