usb: phy: nop: Defer clock prepare until PHY init

Since we only enable the PHY clock on init and the PHY init and shutdown
does not occur in atomitc context there is no need to prepare the clock
before it is enabled.  Move the clk_prepare() operations to go along
with the enables, allowing the clock to be fully idle when not in use.

Signed-off-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>