ALSA: seq: Fix init order of snd_seq_device stuff

When the sequencer driver is built in kernel, it may panic at boot
because of the uninitialized snd_seq_bus_type.  Initialize it properly
via subsys_initcall() instead of module_init() to assure that the bus
is registered beforehand.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Fixes: 7c37ae5c ('ALSA: seq: Rewrite sequencer device binding with standard bus')
Signed-off-by: Takashi Iwai <tiwai@suse.de>