usb: chipidea: imx: refine clock operations to adapt for all platforms

Some i.mx platforms need three clocks to let controller work, but
others only need one, refine clock operation to adapt for all
platforms, it fixes a regression found at i.mx27.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>
Cc: <stable@vger.kernel.org> #v4.1+