first_stage_mount: Fix log typo "check_at_most_once"

Was "check_most_at_once", should be "check_at_most_once".
Also straighten the if-then-else logic from:
  if (cond) {
    // then
  } else {
    // error...
    return;
  }
to:
  if (!cond) {
    // error...
    return;
  }
  // fallthrough is "then"

Test: Presubmit
Change-Id: I85c1e94c47a727089374d5d05ecd40f4187b66a1