ARM: shmobile: defconfig: Replace USB_XHCI_RCAR by USB_XHCI_PLATFORM

USB_XHCI_RCAR is no longer enabled, as it now has a dependency on
USB_XHCI_PLATFORM, instead of selecting USB_XHCI_PLATFORM.
As the latter selects the former if ARCH_RENESAS, fix this by replacing
USB_XHCI_RCAR by USB_XHCI_PLATFORM in shmobile_defconfig.

Fixes: f879fc32 ("usb: host: xhci-rcar: Avoid long wait in xhci_reset()")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>