soc: ixp4xx: qmgr: Fix an NULL vs IS_ERR() check in probe

The devm_ioremap_resource() function doesn't retunr NULL, it returns
error pointers.

Fixes: ecc133c6 ("soc: ixp4xx: qmgr: Pass resources")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>