When the FIFO is empty, returns the last read entry not the next one.
also rewrite the FIFO index increment to generate slightly better code.
Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
BUG=chrome-os-partner:8869
TEST=On Snow, in U-Boot using "stdin=mkbp-keyb" type on internal
keyboard and see the correct text.
Change-Id: I189d230053de40dd563ce672db82dd6217e545e3