serial: stm32: BRR must be set only when usart is disable

To avoid spurious chars, BRR register must only be written when
USART is disabled.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
1 file changed