There's a read-only table for each sampling rate, while it doesn't have
const qualifier and can be modified.

This commit add the qualifier. As a result, a symbol for the table
moves from .data section to .rodata.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>