Commit b7b13588 changed the internal flush time tracking, but
introduced the possibility of motion restart occurring too close to
the last motion end in some rare cases. This could result in
internal stepcompress errors.
Track the last step generation flush time (last_sg_flush_time) and use
when recalculating the next print_time.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>