PCI: endpoint: Use IS_ERR_OR_NULL() helper function

Use the IS_ERR_OR_NULL() helper instead of open-coding a NULL and an
error pointer checks to simplify the code and improve readability.

No functional changes are intended.

[kwilczynski: commit log]
Signed-off-by: Ruan Jinjie <ruanjinjie@huawei.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>