p9221_charger: fix implicit fall-through error

Fixes the following -Wimplicit-fallthrough error:

  bms/p9221_charger.c:2285:2:
  error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
        default:
        ^
  bms/p9221_charger.c:2285:2:
  note: insert 'break;' to avoid fall-through
        default:
        ^
        break;

Bug: 215617713
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Change-Id: I2b1f52bbfeb7a6c6aca049a381a481de867651a5