ALSA: rawmidi - Add open check in rawmidi callbacks

The drivers (e.g. mtpav) may call rawmidi functions in irq handlers
even though the streams are not opened.  This results in Oops or panic.

This patch adds the rawmidi state check before actually operating the
rawmidi buffers.

Tested-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>