[ALSA] Remove ifdefs from OSS PCM emulation codes

Fix Makefile to compile files conditionally to CONFIG_SND_PCM_OSS_PLUGINS,
and remove unneeded ifdefs in these files.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>