PCI: endpoint: pci-epf-test: Refactor pci_epf_test_unbind() function

PCI: endpoint: pci-epf-test: Refactor pci_epf_test_unbind() function

Move the pci_epc_clear_bar() and pci_epf_free_space() code to respective
helper functions. This allows reusing the helpers in future commits.

This also requires moving the pci_epf_test_unbind() definition below
pci_epf_test_bind() to avoid forward declaration of the above helpers.

No functional change.

Link: https://lore.kernel.org/linux-pci/20240430-pci-epf-rework-v4-5-22832d0d456f@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>