max77729_charger: fix implicit fall-through error

Fixes the following -Wimplicit-fallthrough error:

  bms/max77729_charger.c:752:2:
  error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
        case CHGR_DTLS_TIMER_FAULT_MODE:
        ^
  bms/max77729_charger.c:752:2:
  note: insert '__attribute__((fallthrough));' to silence this warning
        case CHGR_DTLS_TIMER_FAULT_MODE:
        ^
        __attribute__((fallthrough));
  bms/max77729_charger.c:752:2:
  note: insert 'break;' to avoid fall-through
        case CHGR_DTLS_TIMER_FAULT_MODE:
        ^
        break;

Bug: 215617713
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Change-Id: If06e379f95342b0a48fa3cf6406ec957dd341401