tf700t: set mnt_flags to defaults

* mnt_flags: Vold ignores this field and it should be set to 'defaults',
  http://source.android.com/devices/tech/storage/index.html. Just setting the
  mnt_flags option to 'defaults', rather than 'default'.

Change-Id: Id23c2715338ed27b6b6b40b4c50b2ad380983723