snd-pcsp: depend on CONFIG_EXPERIMENTAL

Considering all the feedbacks I got, depending snd-pcsp on
CONFIG_EXPERIMENTAL looks like the only safe way to get out
of all the troubles at one go. :)

Signed-off-by: Stas Sergeev <stsp@aknet.ru>
Signed-off-by: Takashi Iwai <tiwai@suse.de>