g: fix UART TX done logic
g: fix UART TX done logic

TX is done when both TX_IDLE and TX_EMPTY conditions are true.

Fixing the check makes unnecessary the code which waited for another
character time before proceeding when flushing the UART TX FIFO.

BRANCH=cr50, cr50-mp
BUG=b:140305442
TEST=added code to print a really long string before reset in the
    'reboot' command, observed that the entire string is reliably
    printed before the reset.

Change-Id: I0882d96ba9ca5412deb704ccdbc43e8cebeeeab5
Signed-off-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1779587
Reviewed-by: Marius Schilder <d423f2980be0298efdef7df239b147d21d472f26@chromium.org>
Reviewed-by: Andrey Pronin <53bbebe6910b11f7aa2d1daaa466658eb4cba5f2@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec