ipmi/powernv: Fix error return code in ipmi_powernv_probe()

Fix to return a negative error code from the request_irq() error
handling case instead of 0, as done elsewhere in this function.

Fixes: dce143c3

 ("ipmi/powernv: Convert to irq event interface")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Corey Minyard <cminyard@mvista.com>