ALSA: Fix forgotten dependency fix for tristate OSS sequencer kconfig

In the commit 3d774d5e ("ALSA: seq: Allow the tristate build of
OSS emulation") we changed CONFIG_SND_SEQUENCER_OSS to tristate, but a
couple of places were forgotten, namely, opl3 and emux Makefile.
These contain the line like
  snd-opl3-synth-$(CONFIG_SND_SEQUENCER_OSS) += opl3_oss.o
and this doesn't work any longer as expected because snd-opl3-synth
can be built-in while CONFIG_SND_SEQUENCER_OSS=m.

This patch fixes these places to build properly for the new kconfig
dependency.  In the end, we had to use ifneq() to satisfy the
requirement.  It's a bit ugly, but lesser evil.

Fixes: 3d774d5e

 ("ALSA: seq: Allow the tristate build of OSS emulation")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>