fpga: dfl: fme: fix return value check in in pr_mgmt_init()

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

Fixes: 29de7624 ("fpga: dfl: fme: add partial reconfiguration sub feature support")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Moritz Fischer <mdf@kernel.org>
Acked-by: Alan Tull <atull@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>