PCI: endpoint: pci-ep-msi: Fix NULL vs IS_ERR() check in pci_epf_write_msi_msg()

The pci_epc_get() function returns error pointers. It never returns NULL.
Update the check to match.

Fixes: 1c3b002c ("PCI: endpoint: Add RC-to-EP doorbell support using platform MSI controller")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/aIzCdV8jyBeql-Oa@stanley.mountain