ftdiuart: bugfix for buffer pointer on partial write to FD.

Previously I neglected to increment the buffer pointer which allow me
to double write same data and exclude tail if write didn't complete in
one shot.

Also fixing bug with EAGAIN/EWOULDBLOCK to retry in those cases.
Previously we'd just have quitely ignored the write.

BUG=chromium:371147
TEST=manual
- start servod on board with active uart
- send & receive uart traffic successfully

Change-Id: I0d8b0a2f87346c3667c711f0c2ebb491cefcd5ae
Reviewed-on: https://chromium-review.googlesource.com/198867
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>
Commit-Queue: Todd Broch <tbroch@chromium.org>
Tested-by: Todd Broch <tbroch@chromium.org>
1 file changed