stm32: make SPI CLK fast to fix data issue

See "STM32F40x and STM32F41x Errata sheet" - SPI CLK port must be 'fast' or 'very fast' to avoid data corruption on last bit (at the APB clocks we configure).

src/machine/machine_stm32_moder_gpio.go

@@ -55,7 +55,9 @@ const (
 	gpioPullMask     = 0x3
 
 	// OSPEED bitfields.
+	gpioOutputSpeedVeryHigh = 3
 	gpioOutputSpeedHigh     = 2
+	gpioOutputSpeedMedium   = 1
 	gpioOutputSpeedLow      = 0
 	gpioOutputSpeedMask     = 0x3
 )
@@ -120,7 +122,7 @@ func (p Pin) ConfigureAltFunc(config PinConfig, altFunc uint8) {
 	// SPI
 	case PinModeSPICLK:
 		port.MODER.ReplaceBits(gpioModeAlternate, gpioModeMask, pos)
-		port.OSPEEDR.ReplaceBits(gpioOutputSpeedLow, gpioOutputSpeedMask, pos)
+		port.OSPEEDR.ReplaceBits(gpioOutputSpeedHigh, gpioOutputSpeedMask, pos)
 		port.PUPDR.ReplaceBits(gpioPullFloating, gpioPullMask, pos)
 		p.SetAltFunc(altFunc)
 	case PinModeSPISDO: