sound: rawmidi: fix double init when opening MIDI device with O_APPEND

sound: rawmidi: fix double init when opening MIDI device with O_APPEND

Commit 3c4c7477def325abd54b15e2b08b3dac7bb8f5f4 in 2.6.30 moved the
substream initialization code to where it would be executed every time
the substream is opened.

This had the consequence that any further opening would drop and leak
the data in the existing buffer, and that the device driver's open
callback would be called multiple times, unexpectedly.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>