add comment for snd_pcm_hw_constraint_msbits

It's related to hardware restriction. When the hardware DSP can process 32 bit sample but
ignore its lower 8bit, this constraint is used.
http://www.alsa-project.org/alsa-doc/alsa-lib/pcm.html#pcm_formats
rename snd_efw_create_midi_ports to snd_efw_create_midi_devices

For consistency of naming.