Keyborg: increase UART baud rate to 38400
Keyborg: increase UART baud rate to 38400

We are running a minimal runtime with less overhead. This allows us to
run UART at 38400 bps. Let's update the config for easier debugging.

Also fix a potential underflow bug.

BUG=None
TEST=See debug output at 38400 bps
BRANCH=None

Change-Id: Ic9e4f9d545f5dbc4a0816a843b0f01a4cf219666
Signed-off-by: Vic (Chun-Ju) Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/196190
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

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