MIDI in doesn't work on RP2040 at F_CPU=150MHz

[dpwe]

MIDI in works at all available clock rates except 150MHz.

If I run the cpu clock adjustment in pico_support.cpp:pico_setup_i2s() before I configure the UART in amy_midi.c:run_midi(), this works. But currently run_midi is part of amy_start(), and pico_setup_i2s is part of amy_live_start(), which has to be run after amy_start.

I'm planning on eliminating amy_live_start (in #489) and instead controlling whether AMY tries to drive the I2S in the background via flags in config. So I can probably fix this ordering at that time also.