Currently, Spawn.expect() imposes its timeout solely upon receipt of new
data, not on its overall operation. In theory, this could cause the
timeout not to fire if U-Boot continually generated output that did not
match the expected patterns.

Fix the code to additionally impose a timeout on overall operation, which
is the intended mode of operation.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>