samd: diagnose out of range I2C frequency

The frequency divisor is limited to 255, which gives 48MHz/2/255 ~= 94.1kHz as
the lowest speed.

Without this change, values below this cut-off gave higher frequencies instead,
which didn't appear to have any relation to the frequency value requested.

Closes: #4883

Author: jepler

ports/atmel-samd/common-hal/busio/I2C.c

@@ -119,6 +119,13 @@ void common_hal_busio_i2c_construct(busio_i2c_obj_t *self,
     // clkrate is always 0. baud_rate is in kHz.
 
     // Frequency must be set before the I2C device is enabled.
+    // The maximum frequency divisor gives a clock rate of around 48MHz/2/255
+    // but set_baudrate does not diagnose this problem. (This is not the
+    // exact cutoff, but no frequency well under 100kHz is available)
+    if (frequency < 95000) {
+        mp_raise_ValueError(translate("Unsupported baudrate"));
+    }
+
     if (i2c_m_sync_set_baudrate(&self->i2c_desc, 0, frequency / 1000) != ERR_NONE) {
         reset_pin_number(sda->number);
         reset_pin_number(scl->number);