ntb: idt: fix error check in ntb_hw_idt.c

idt_create_dev never return NULL and fix smatch warning.

Signed-off-by: Wang Qing <wangqing@vivo.com>
Acked-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>