drivers: uart_mcux_flexcomm: fix dma rx config

The DMA configurations for TX and RX  were mixed-up
letting the DMA RX channel not fully configured.

This fix correctly configures the DMA RX channel
with DMA_ADDR_ADJ_NO_CHANGE.

Signed-off-by: Rémy Dziemiaszko <remy.dziemiaszko@smile.fr>