STM32: USART: Fix baudrate calculation for LPUART1

LPUART1 needs BRR to be 256 times the normal value.

Doing this with 64 bit math would work, but that pulls in quite a lot of
code for every platform, even if they don't have LPUART.

A little bit of maths, thanks to zyp on irc, lets us keep it in 32bit,
provided people don't try and use > 16MBaud on LPUARTs

Originally tracked via https://github.com/libopencm3/libopencm3/pull/888

lib/stm32/common/usart_common_all.c

@@ -42,6 +42,9 @@ USART1/6) or the APB low-speed prescaler clock (for other USARTs). These values
 must be correctly set before calling this function (refer to the
 rcc_clock_setup-* functions in RCC).
 
+Note: For LPUART, baudrates over 2**24 (~16.7 Mbaud) may overflow
+the calculation and are therefore not supported by this function.
+
 @param[in] usart unsigned 32 bit. USART block register address base @ref
 usart_reg_base
 @param[in] baud unsigned 32 bit. Baud rate specified in Hz.
@@ -70,7 +73,15 @@ void usart_set_baudrate(uint32_t usart, uint32_t baud)
 	 * Note: We round() the value rather than floor()ing it, for more
 	 * accurate divisor selection.
 	 */
-	USART_BRR(usart) = ((2 * clock) + baud) / (2 * baud);
+#ifdef LPUART1
+	if (usart == LPUART1) {
+		USART_BRR(usart) = (clock / baud) * 256
+			+ ((clock % baud) * 256 + baud / 2) / baud;
+		return;
+	}
+#endif
+
+	USART_BRR(usart) = (clock + baud / 2) / baud;
 }
 
 /*---------------------------------------------------------------------------*/