ALSA: make snd_ctl_elem_read() and snd_ctl_elem_write() static

snd_ctl_elem_read() and snd_ctl_elem_write() are no longer used by
any other drivers.

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