m4: fix OTG mountpoint in fstab

vold complains:
E/DirectVolume( 234): Vold managed volumes must have auto mount point;
ignoring /storage/usbdisk

Caused by fstab format change between 4.3 and 4.4. Boot tested only, I
don't have any OTG equipment.

Change-Id: I7fbc8c523a56d98a701f34e44e93855283b5b217