serial: stm32f7: disable overrun

With overrun enabled, serial port console freezes & stops receiving data with
overun error if we keep sending data.

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>
diff --git a/drivers/serial/serial_stm32x7.h b/drivers/serial/serial_stm32x7.h
index 6190d67..8c02548 100644
--- a/drivers/serial/serial_stm32x7.h
+++ b/drivers/serial/serial_stm32x7.h
@@ -27,6 +27,8 @@
 #define USART_CR1_TE			(1 << 3)
 #define USART_CR1_UE			(1 << 0)
 
+#define USART_CR3_OVRDIS		(1 << 12)
+
 #define USART_SR_FLAG_RXNE		(1 << 5)
 #define USART_SR_FLAG_TXE		(1 << 7)