driver: uart: npcx: avoid concurrency writing UFTCTL register

In order to avoid concurrency situation during writing UFTCTL register,
this CL adds critical sections to prevent the unexpected result if the
driver wants to set/clear bits of this register.

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