Prepare the clock before enabling it.

Cc: <linux-usb@vger.kernel.org>
Signed-off-by: NFabio Estevam <fabio.estevam@freescale.com>
Acked-by: NSascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: NFelipe Balbi <balbi@ti.com>