greybus: loopback_test: Use timeout argument

Patch "c3b0a32 Loopback_test: use poll instead of inotify"
added a optional argument for the user to specify a timeout value,
but did not use this parameter in the actual poll function. The
default of 30 seconds is always used.

Fix this by actually using the the poll_timeout parameter so the user
can run long tests.

Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Reviewed-by: Patrick Titiano <ptitiano@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>