Fix GPIO alternate function definition

A change of the mode of a GPIO to Mode::ALTERNATE before setting the alternate function number could cause a glitch where the GPIO is initialized with an undefined alternate function before changing it to the wanted one. This error can be found in a lot of bsp.cpp files and in drivers that change the mode of the pin (e.g. I2C).

TL;DR: This is an error:

pin.mode(Mode::ALTERNATE);
pin.alternateFunction(<alternateFunctionNumber>);

This is right:

pin.alternateFunction(<alternateFunctionNumber>);
pin.mode(Mode::ALTERNATE);