ocxl: Fix return value check in afu_ioctl()

In case of error, the function eventfd_ctx_fdget() returns ERR_PTR() and
never returns NULL. The NULL test in the return value check should be
replaced with IS_ERR().

This issue was detected by using the Coccinelle software.

Fixes: 06014661 ("ocxl: move event_fd handling to frontend")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Alastair D'Silva <alastair@d-silva.org>
Acked-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>