powerpc/4xx: Fix return value check in hsta_msi_probe()

In case of error, the function ioremap() returns NULL
not ERR_PTR(). The IS_ERR() test in the return value
check should be replaced with NULL test.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>