snd-pcsp: adjust help texts to frighten users

Added the warning text to the help of snd-pcsp about the possible problem
with this driver so that user can know of the problem in advance.

Also, removed the obsoleted text about ancient pc-speaker patch in
CONFIG_SOUND help.

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