Add wall clock timeout warning to mpsse_flush()

Add wall clock timeout warning to mpsse_flush()

I think that libusb_handle_events_timeout_completed is supposed to make
progress or time out, but sometimes we hit a case where it makes no
progress, and mpsse_flush() loops forever. This wall clock timeout
notifies the user that this is going on.

When I wrote this code, this bug would reproduce every hour or two, but
right now it's not happening for me.

Change-Id: I7eb66f43462298e263a48048aa0c8769095661eb
Signed-off-by: Tim Newsome <tim@sifive.com>
Reviewed-on: http://openocd.zylin.com/4767
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>