ALSA: seq: Allow the modular sequencer registration

Many drivers bind the sequencer stuff in off-load by another driver
module, so that it's loaded only on demand.  In the current code, this
mechanism doesn't work when the driver is built-in while the sequencer
is module.  We check with IS_REACHABLE() and enable only when the
sequencer is in the same level of build.

However, this is basically a overshoot.  The binder code
(snd-seq-device) is an individual module from the sequencer core
(snd-seq), and we just have to make the former a built-in while
keeping the latter a module for allowing the scenario like the above.

This patch achieves that by rewriting Kconfig slightly.  Now, a driver
that provides the manual sequencer device binding should select
CONFIG_SND_SEQ_DEVICE in a way as
	select SND_SEQ_DEVICE if SND_SEQUENCER != n

Note that the "!=n" is needed here to avoid the influence of the
sequencer core is module while the driver is built-in.

Also, since rawmidi.o may be linked with snd_seq_device.o when
built-in, we have to shuffle the code to make the linker happy.
(the kernel linker isn't smart enough yet to handle such a case.)
That is, snd_seq_device.c is moved to sound/core from sound/core/seq,
as well as Makefile.

Last but not least, the patch replaces the code using IS_REACHABLE()
with IS_ENABLED(), since now the condition meets always when enabled.

Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/isa/sb/emu8000.c

@@ -1138,7 +1138,7 @@ snd_emu8000_new(struct snd_card *card, int index, long port, int seq_ports,
 		snd_emu8000_free(hw);
 		return err;
 	}
-#if IS_REACHABLE(CONFIG_SND_SEQUENCER)
+#if IS_ENABLED(CONFIG_SND_SEQUENCER)
 	if (snd_seq_device_new(card, index, SNDRV_SEQ_DEV_ID_EMU8000,
 			       sizeof(struct snd_emu8000*), &awe) >= 0) {
 		strcpy(awe->name, "EMU-8000");