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

commit ae3e57ae upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>