stm32l4 tty reconfigures uart periphery without disabling DMA channel

[xvuko]

RM0351 Reference manual Rev 10
40.8.1 USART control register 1 (USART_CR1)

The DMA requests are also reset when UE = 0 so the DMA channel must be disabled
before resetting the UE bit

Currently set_baudrate callback called from libtty ioctl hander temporarily disables UE without any synchronization. This needs to be investigated as it can lead to race conditions even when DMA is not used.

[xvuko]

For libuart this was addressed in phoenix-rtos/phoenix-rtos-devices@d1fd028