drivers: serial: ra_sci: add missing `break` in `callback_adapter`

Added missing `break` statement in `uart_ra_sci_callback_adapter`
for the `UART_EVENT_RX_COMPLETE` case.
Without it, `async_evt_rx_err()` would be called, which is not
the desired operation.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>