drivers: watchdog: npcx: mask the WDRST_STS when accessing T0CSR

Register T0CSR bit 4 is WDRST_STS which is used to check whether the
chip has watchdog reset from the last power-up or vcc1_rst. WDRST_STS
hardware is design to write one clear. For the original
read-modify-write, it will reset the WDRST_STS unexpected. Add a mask
to avoid it.

Signed-off-by: Wealian Liao <WHLIAO@nuvoton.com>