driver: itim: npcx: check ITEN bit to prevent return fake error.

During polling ITEN bit to make sure ITIM timer is enabled, we might
have the chance that npcx_itim_evt_enable() return fake error when
timeout expired but ITEN bit is set already if CONFIG_ZERO_LATENCY_IRQS
is enabled. (Since SVCall's interrupt priority is not the highest, the
other interrupts with IRQ_ZERO_LATENCY flag could preempt CPU resource
at this moment.)

In order to prevent return fake error code, this CL adjusts the check
conditions for ITEN bit and timeout.

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