recovery: Fix the FIXME in get_menu_selection().

It used to return a REBOOT action on timeout, until the CL in commit
daefc1d4 that redefined the return value
of get_menu_selection() (changing from action to a menu index).

Prior to this CL, it was returning 0 (i.e. the value of Device::REBOOT)
to trigger the reboot. This CL specifies a return value of -1 to
indicate the timeout.

Test: Boot into a user build recovery; it reboots automatically on
      timeout (120 sec).
Change-Id: I4aedb7a4628bf258017078fe73eb8b48a21d0ea8