mfd: Fix omap_usbhs_alloc_children error handling

1. Return proper error if omap_usbhs_alloc_child fails
2. In the case of goto err_ehci, we should call platform_device_unregister(ehci)
   instead of platform_device_put(ehci) because we have already added the
   platform device to device hierarchy.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Tested-by: Keshava Munegowda <keshava_mgowda@ti.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>