drivers: serial: uart_mcux_lpuart: Restore lpuart CTRL register.

When RTS/CTS is toggled it disables the receiver and transmitter
bits to be able to change the flow control bits in the MODIR register.

After it has done so, it should restore the original value of the
CTRL register to re-enable the transmitter and/or receiver if they
were enabled before calling the mcux_lpuart_line_ctrl_set_rts()
function.

Signed-off-by: Bas van Loon <bas@arch-embedded.com>