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]