[ALSA] snd-ak4114: Fix two array overflows

Fix the handling of the TXCSB registers cache. There was one array
overflow in reg_write() and one in snd_ak4114_reg_write(). Thanks
to David Binderman for reporting the latter.
The second overflow probably doesn't matter much, given that the
function snd_ak4114_reg_write() appears to be never called. I wonder
why it exists and why it is exported.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>