staging: line6: rename MidiBuffer to avoid CamelCase

Fix checkpatch.pl warnings related to MidiBuffer:

  WARNING: Avoid CamelCase: <MidiBuffer>
  #947: FILE: staging/line6/driver.c:363:
  +       struct MidiBuffer *mb = &line6->line6midi->midibuf_in;

Rename MidiBuffer to midi_buffer.

Note that "midibuf" would be another good name but sound/oss/midibuf.c
already uses it for a different concept.  Avoid possible confusion by
using "midi_buffer" instead.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>