drivers: serial: Fix "mcux_lpuart_irq_tx_complete" returns wrong result

Use "kLPUART_TransmissionCompleteFlag" instead of
"kLPUART_TxDataRegEmptyFlag" to check if the tx transmition is finished.

The "kLPUART_TxDataRegEmptyFlag" would give a wrong result
even if the transmition isn't over.

Signed-off-by: Andy Liu <andy@madmachine.io>