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

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.

Fixes: 1bf5f25324f7 ("PCI: endpoint: Add PCI Endpoint function driver for MHI bus")
Link: https://lore.kernel.org/r/20241105120735.1240728-1-quic_zhonhan@quicinc.com
Signed-off-by: Zhongqiu Han <quic_zhonhan@quicinc.com>
[kwilczynski: error message update per the review feedback]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
[bhelgaas: commit log]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Niklas Cassel <cassel@kernel.org>