google_cpm: fix implicit fall-through errors

Fixes several -Wimplicit-fallthrough errors similar to the following:

  bms/google_cpm.c:465:2:
  error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
        case DC_ENABLE:
        ^
  bms/google_cpm.c:465:2:
  note: insert '__attribute__((fallthrough));' to silence this warning
        case DC_ENABLE:
        ^
        __attribute__((fallthrough));
  bms/google_cpm.c:465:2:
  note: insert 'break;' to avoid fall-through
        case DC_ENABLE:
        ^
        break;

Bug: 215617713
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Change-Id: I622ac02de9bfacf7cffc716c3aef12dd4780159d