stm32/spi: Fix baudrate calculation for H7 series.
stm32/spi: Fix baudrate calculation for H7 series.

Fixes issue #6342.

adafruit/circuitpython,henriknelson/micropython,henriknelson/micropython,adafruit/circuitpython,bvernoux/micropython,henriknelson/micropython,adafruit/circuitpython,bvernoux/micropython,adafruit/circuitpython,henriknelson/micropython,bvernoux/micropython,bvernoux/micropython,bvernoux/micropython,henriknelson/micropython,adafruit/circuitpython,adafruit/circuitpython