edgetpu: fix implicit fall-through error

Fixes the following -Wimplicit-fallthrough error:

  edgetpu/drivers/edgetpu/abrolhos-pm.c:423:2:
  error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
        default:
        ^
  edgetpu/drivers/edgetpu/abrolhos-pm.c:423:2:
  note: insert '__attribute__((fallthrough));' to silence this warning
        default:
        ^
        __attribute__((fallthrough));
  edgetpu/drivers/edgetpu/abrolhos-pm.c:423:2:
  note: insert 'break;' to avoid fall-through
        default:
        ^
        break;

Bug: 215617713
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Change-Id: Ic682ec12c7b6a8eab38d3fc5b253d26b3a581d30