ITE drivers/sensor/vcmp: don't connect shared irq multiple times

This driver exectue connecting the shared irq one time for
every status "okay" vcmp nodes, then it will show enable
the same irq multiple times when build.

So I check whether the irq is enabled or not, if yes,
we needn't to enable again. And we will figure out the
triggered channel in vcmp_it8xxx2_isr().

Signed-off-by: Ruibin Chang <Ruibin.Chang@ite.com.tw>