ntb: idt: Fix the cacography in ntb_hw_idt.c

The word 'swtich' is wrong, so fix it.

Signed-off-by: zhang jiao <zhangjiao2@cmss.chinamobile.com>
Acked-by: Serge Semin <fancer.lancer@gmail.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>