usb: phy: phy-generic: propagate clk_get error if clock is required

If the clock handle is given in the DT, it means the clock is required
for proper operation of the PHY. In that case a failure to obtain the
clock must be propagated to stop the driver from probing. This fixes
working with clocks, which request probe deferral.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>