ALSA: Make CONFIG_SND_OSSEMUL user-selectable

Currently CONFIG_SND_OSSEMUL is selected by each config like
CONFIG_SND_PCM_OSS.  But, as see in the raw MIDI code that is built
conditionally with CONFIG_SND_OSSEMUL, we should rather make
CONFIG_SND_OSSEMUL user-selectable as the top kconfig item, and leave
the rest depending on it.

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