usb: xhci: avoid renesas_usb_fw.mem when it's unusable

Commit a66d21d7 ("usb: xhci: Add support for Renesas controller with
memory") added renesas_usb_fw.mem firmware reference to xhci-pci.  Thus
modinfo indicates xhci-pci.ko has "firmware: renesas_usb_fw.mem".  But
the firmware is only actually used with CONFIG_USB_XHCI_PCI_RENESAS.  An
unusable firmware reference can trigger safety checkers which look for
drivers with unmet firmware dependencies.

Avoid referring to renesas_usb_fw.mem in circumstances when it cannot be
loaded (when CONFIG_USB_XHCI_PCI_RENESAS isn't set).

Fixes: a66d21d7

 ("usb: xhci: Add support for Renesas controller with memory")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Thelen <gthelen@google.com>
Link: https://lore.kernel.org/r/20210702071224.3673568-1-gthelen@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>