usb: gadget: r8a66597-udc: convert to clk_prepare/unprepare

Turn clk_enable() and clk_disable() calls into clk_prepare_enable() and
clk_disable_unprepare() to get ready for the migration to the common
clock framework.

Cc: Felipe Balbi <balbi@ti.com>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>