stm32/pin_defs_stm32: Fix pin printing to show IN mode correctly.

Prior to this commit, if you configure a pin as an output type (I2C in this
example) and then later configure it back as an input, then it will report
the type incorrectly.  Example:

    >>> import machine
    >>> b6 = machine.Pin('B6')
    >>> b6
    Pin(Pin.cpu.B6, mode=Pin.IN)
    >>> machine.I2C(1)
    I2C(1, scl=B6, sda=B7, freq=420000)
    >>> b6
    Pin(Pin.cpu.B6, mode=Pin.ALT_OPEN_DRAIN, pull=Pin.PULL_UP, af=Pin.AF4_I2C1)
    >>> b6.init(machine.Pin.IN)
    >>> b6
    Pin(Pin.cpu.B6, mode=Pin.ALT_OPEN_DRAIN, af=Pin.AF4_I2C1)

With this commit the last print now works:

    >>> b6
    Pin(Pin.cpu.B6, mode=Pin.IN)

ports/stm32/pin_defs_stm32.c

@@ -8,9 +8,9 @@
 uint32_t pin_get_mode(const pin_obj_t *pin) {
     GPIO_TypeDef *gpio = pin->gpio;
     uint32_t mode = (gpio->MODER >> (pin->pin * 2)) & 3;
-    if (mode != GPIO_MODE_ANALOG) {
+    if (mode == GPIO_MODE_OUTPUT_PP || mode == GPIO_MODE_AF_PP) {
         if (gpio->OTYPER & pin->pin_mask) {
-            mode |= 1 << 4;
+            mode |= 1 << 4;     // Converts from xxx_PP to xxx_OD
         }
     }
     return mode;