ALSA: oxfw: retry MIDI transferring for scs1x at transaction failure

Currently, ALSA oxfw driver has a TODO to retry MIDI transferring
at transaction failure.

This commit achieves it. Current implementation uses snd_rawmidi_transmit()
to transfer messages, thus the target MIDI messages are not in buffer when
transaction failure is detected. Although we cannot use a pair of
snd_rawmidi_transmit_peek() and snd_ramwidi_transmit_ack(), the
messages are still in scs1x specific structure and the data is available
for retries.

This commit adds a member to the structure for the length of buffered
messages, and uses the value again at retries.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>