mailbox/xgene-slimpro: Checking for IS_ERR instead of NULL

devm_ioremap() returns NULL, it never returns an ERR_PTR.

Fixes: f700e84f ('mailbox: Add support for APM X-Gene platform mailbox driver')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>