This commit copies some functions of asynchronous transactions for MIDI
capture, to merge scs1x module. The features of payload in asynchronous
transaction are:

 * System exclusive messages for SCS.1 are encoded without ID data. In
   this encoding scheme, 4 bits in LSB are available. The bits are squashed
   in payload byte. Thus, one payload byte transfers two MIDI messages.
 * The first byte of payload byte means:
  * 0x00: depending on second payload byte
   * 0xf9: including escaped system exclusive messages for SCS.1, up to
     3 byte (= 6 MIDI messages)
   * the others: including MIDI 1.0 messages
  * the others: including escaped system exclusive messages for SCS.1, up
    to 64 bytes
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>