pty_driver: Don't assert if no response is read after flush

Our flush operation involves sending a newline to the pty and then
reading a response back. If our UART buffer is empty and the remote
device is taking longer than expected to respond, print a debug
statement rather than asserting. This change is consistent with our
existing error handling.

BUG=chrome-os-partner:40825
TEST=Manual on Rambi. Empty UART buffer, verify that "dut-control
power_state:rec" puts the device into recovery mode.

Change-Id: I1e47db824d1d1a44d973d895871a728d44dced47
Signed-off-by: Shawn Nematbakhsh <shawnn@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/280905
Reviewed-by: Todd Broch <tbroch@chromium.org>
1 file changed