ALSA: seq-oss: Initialize MIDI clients asynchronously

We've got bug reports that the module loading stuck on Debian system
with 3.10 kernel.  The debugging session revealed that the initial
registration of OSS sequencer clients stuck at module loading time,
which involves again with request_module() at the init phase.  This is
triggered only by special --install stuff Debian is using, but it's
still not good to have such loops.

As a workaround, call the registration part asynchronously.  This is a
better approach irrespective of the hang fix, in anyway.

Reported-and-tested-by: Philipp Matthias Hahn <pmhahn@pmhahn.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>