usb: host: xhci-rcar: Add a helper macro to set xhci_plat_priv

To avoid copy-and-paste setting of xhci_plat_priv for R-Car SoCs,
this patch add a helper macro SET_XHCI_PLAT_PRIV_FOR_RCAR.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/1567425698-27560-3-git-send-email-yoshihiro.shimoda.uh@renesas.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>