usb: phy: phy-generic: Check clk_prepare_enable() error

clk_prepare_enable() may fail, so we should better check its return
value and propagate it in the case of failure.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>