driver: gpio: npcx: correct the usage of npcx voltage flags

In npcx ec series, two detection levels, 3.3V (default) and 1.8V are
supported during gpio configuration. But the current implementation
always selects default detection level whether NPCX_GPIO_VOLTAGE_1P8 is
set. This PR is a fix for this issue.

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