update lib to latest, and fix for changes

rcc_core_frequency -> rcc_ahb_frequency
update lib to latest, and fix for changes

rcc_core_frequency -> rcc_ahb_frequency
stm32f4: usb_midi: Introduce example MIDI firmware

Tested on STM32F4Discovery with Linux host using both amidi (raw
messages to/from device) and a synthesizer (note on/off) and on
Windows 7 using MIDI-OX.