usb: imx21-hcd.c: Use clk_prepare_enable/clk_disable_unprepare

Prepare the clock before enabling it.

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