usb: host: xhci-rcar: Add a condition check about PLL active

This patch adds a condition check about the PLL acvice of this
controller. Otherwise, the controller might cause hang when
any USB clocks are not supplied yet and accesses the xHCI registers.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>