mailbox: zynqmp-ipi: Fix NULL vs IS_ERR() check in zynqmp_ipi_mbox_probe()

In case of error, the function devm_ioremap() returns NULL pointer not
ERR_PTR(). So we should check whether the return value of devm_ioremap()
is NULL instead of IS_ERR.

Fixes: 4981b82b

 ("mailbox: ZynqMP IPI mailbox controller")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>