usb: host: xhci: plat: re-fact xhci_plat_priv for R-Car Gen3

Since the firmware_name is decided by xhci-rcar.c on R-Car Gen3 now,
this patch removes 2 things:
 - Remove struct xhci_plat_priv xhci_plat_renesas_rcar_r8a7796.
 - Remoce .firmware_name from xhci_plat_renesas_rcar_gen3.

The behavior is the same as before.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>