drivers: uart stm32 F4X, F1X, F2X have a sw sequence to clear error flags

Gives more explanation on uart_stm32_err_check function.
On stm32 F4X, F1X, and F2X, when clearing the usart Error Flag
(PE, ORE, FE, NE), the  LL Clear function applies a software sequence
which reads the usart SR then the usart DR.
Consequently the RXNE flag is affected (cleared) by the
uart_stm32_err_check function call.

Signed-off-by: Francois Ramu <francois.ramu@st.com>