mfd: Move omap-usb-host usbhs init before allocing child dev

There could be possible race condition where the host dev's
are alloced and added to platform dev just before usbhs_init.
Just move usbhs_init before adding child dev.

CC: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Govindraj.R <govindraj.raja@ti.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>