PCI: endpoint: Add "volatile" to pci_epf_test_reg

struct pci_epf_test_reg is the MEMSPACE of pci-epf-test function driver
that will be accessed by the "host" for programming the pci-epf-test
device. So this structure shouldn't be subjected to compiler optimization
in pci_epf_test_cmd_handler() since the values can be changed by code
outside the scope of current code at any time.

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