As long as investigating Fireface 400, MIDI messages are transferred by
asynchronous communication over IEEE 1394 bus.

Fireface 400 receives MIDI messages by write transactions to two addresses;
0x'0000'0801'8000 and 0x'0000'0801'9000. Each of two seems to correspond to
MIDI port 1 and 2.

Fireface 400 transfers MIDI messages by write transactions to certain
addresses which configured by drivers. The drivers can decide upper 4 byte
of the addresses by write transactions to 0x'0000'0801'03f4. For the rest
part of the address, drivers can select from below options:
 * 0x'0000'0000
 * 0x'0000'0080
 * 0x'0000'0100
 * 0x'0000'0180

Selected options are represented in register 0x'0000'0801'051c as bit
flags. Due to this mechanism, drivers are restricted to use addresses on
'Memory space' of IEEE 1222, even if transactions to the address have
some side effects.

This commit adds transaction support for MIDI messaging, based on my
assumption that the similar mechanism is used on the other protocols. To
receive asynchronous transactions, the driver allocates a range of address
in 'Memory space'. I apply a strategy to use 0x'0000'0000 as lower 4 byte
of the address. When getting failure from Linux FireWire subsystem, this
driver retries to allocate addresses.

Unfortunately, read transaction to address 0x'0000'0801'051c returns zero
always, however write transactions have effects to the other features such
as status of sampling clock. For this reason, this commit delegates a task
to configure this register to user space applications. The applications
should set 3rd bit in LSB in little endian order.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>