PCI: endpoint: epf-mhi: Avoid NULL dereference if DT lacks 'mmio'

In the Linux kernel, the following vulnerability has been resolved: PCI: endpoint: epf-mhi: Avoid NULL dereference if DT lacks 'mmio' If platform_get_resource_byname() fails and returns NULL because DT lacks an 'mmio' property for the MHI endpoint, dereferencing res->start will cause a NULL pointer access. Add a check to prevent it. [kwilczynski: error message update per the review feedback] [bhelgaas: commit log]