phy: ti: usb2: Fix logic on -EPROBE_DEFER

If clk_get() returns -EPROBE_DEFER then we should just
return instead of falling back to old clock name.

Use clk_prepare_enable() and clk_disable_unprepare() instead
of splitting up prepare/unprepare from enable/disable.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>