driver: Port80: npcx: defer Port80 code sending to workqueue thread

If the host sends Port80 postcodes frequently while EC is busy handling
other tasks, the Port80 FIFO (16-byte depth) might overflow easily,
especially when the host sends the postcode with the 4-byte format.
This change defers the handling and sending (to the upper layer)
postcodes to the system workqueue thread. It can reduce a lot of
(but not all) the overflow case. Also in practice, we usually care
about the latest postcodes. The older codes are not significant to the
developer. This commit also lowers the printing of the overflow warning
to LOG_DEBUG.

Signed-off-by: Jun Lin <CHLin56@nuvoton.com>