PCI: endpoint: pci-epf-{mhi/test}: Move DMA initialization to EPC init callback

To maintain uniformity across EPF drivers, move DMA initialization to EPC
init callback. This will also allow us to deinit DMA during PERST# assert
in the further commits.

For EPC drivers without PERST#, DMA deinit will only happen during driver
unbind.

Link: https://lore.kernel.org/linux-pci/20240430-pci-epf-rework-v4-6-22832d0d456f@linaro.org


Tested-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Niklas Cassel <cassel@kernel.org>