init: Don't run update_sys_usb_config if /data isn't mounted

* When /data is not mounted / not yet decrypted,
  persistent property overrides are not loaded yet
  so persist.sys.usb.config value is the one from default.prop.

  This caused adb to always be disabled on boot even if
  it was enabled in development settings.

Change-Id: Ic2a51591ad6b77f3463cfa91e9d7362410a021f2