HID: prodikeys: make needlessly global symbols static

The following symbols are not referenced outside this file so
there's no need for it to be in the global name space.

  pcmidi_sustained_note_release
  init_sustain_timers
  stop_sustain_timers
  pcmidi_handle_report
  pcmidi_setup_extra_keys
  pcmidi_snd_initialise
  pcmidi_snd_terminate

Make them static.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>