md/raid5: Use correct IS_ERR() variation on pointer check

This fixes a build error on certain architectures, such as ppc64.

Fixes: 6995f0b2("md: takeover should clear unrelated bits")
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Shaohua Li <shli@fb.com>