usb: xhci: make symbols static

usb: xhci: make symbols static

When renesas module is not built, we get compiler warning on xhci driver
with W=1

  CC [M]  drivers/usb/host/xhci-rcar.o
drivers/usb/host/xhci-pci.h:13:5: warning: no previous prototype for ‘renesas_xhci_check_request_fw’ [-Wmissing-prototypes]
 int renesas_xhci_check_request_fw(struct pci_dev *dev,
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/usb/host/xhci-pci.h:19:6: warning: no previous prototype for ‘renesas_xhci_pci_exit’ [-Wmissing-prototypes]
 void renesas_xhci_pci_exit(struct pci_dev *dev) { };
      ^~~~~~~~~~~~~~~~~~~~~

We have defined these symbols when CONFIG_USB_XHCI_PCI_RENESAS is not
defined, but missed making then static.

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: 8bd5741e3145 ("usb: renesas-xhci: Add the renesas xhci driver")
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Link: https://lore.kernel.org/r/20200519093002.1152144-1-vkoul@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>