usb: renesas_usbhs: move pdev_to_priv to global

usbhs_pdev_to_priv function will be used in other files.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>