drivers: uart_mcux_lpuart: move critical function call out of assert()

without the call of LPUART_ClearStatusFlags(), the interrupt can
immediately trigger again, blocking the application.
CONFIG_ASSERTS=n removes the assert() including the call of
LPUART_ClearStatusFlags().
Also removed LPUART_ClearStatusFlags call for unclearable
kLPUART_TransmissionCompleteFlag.

Signed-off-by: Jan Peters <peters@kt-elektronik.de>