driver: serial/uart: npcx: disable rx interrupt at init

In the application, after the sysjump, the RX interrupt might remain
enabled. During initialization, once the IRQ is enabled, the RX
interrupt could be triggered if there is any traffic on the RX line,
potentially causing an interrupt storm.

This change disables the UART RX interrupt at the driver initialization
to prevent this issue from occurring.

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