serial: stm32: Fix bits defines name

Rename USART_ISR_FLAG_xxx bits to USART_ISR_xxx bits and
USART_ICR_OREF to USART_ICR_ORECF in order to match datasheets.
Sort defines by descendant order.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>