misc: pci_endpoint_test: Fix BUG_ON error during pci_disable_msi()

pci_disable_msi() throws a Kernel BUG if the driver has successfully
requested an IRQ and not released it. Fix it here by freeing IRQs before
invoking pci_disable_msi().

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>