Fix USB OTG

* fstab label "sda" didn't match init.qcom.rc label fuse_usbdisk which
  confused vold
* Mount the drive at /storage/usbdisk instead of usbdisk0 since we only
  have one USB port

Change-Id: I300714c7b2413cb3d8fe4e8b438c92f4d0f0000e