ipmi: NPCM7xx KCS BMC: enable interrupt to the host

Original kcs_bmc_npcm7xx.c was missing enabling to send interrupt to the
host on writes to output buffer.
This patch fixes it by setting the bits that enables the generation of
IRQn events by hardware control based on the status of the OBF flag.

Signed-off-by: Avi Fishman <AviFishman70@gmail.com>
Reviewed-by: Haiyue Wang <haiyue.wang@linux.intel.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>