usb: ulpi: ulpi_init should be executed in subsys_initcall

Phy drivers and the ulpi interface providers depend on the
registration of the ulpi bus.  Ulpi registers the bus in
module_init(). This could cause unnecessary bus users'
probe delays. i.e. unnecessary -EPROBE_DEFER happening on
ulpi_drivers in case they're registered before ulpi bus
itself.

Reported-by: Zhuo Qiuxu <qiuxu.zhuo@intel.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Acked-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>