ALSA: mixer: oss: Constify snd_mixer_oss_assign_table definition

The snd_mixer_oss_assign_table is read-only, and can it be declared as
const.

There should be no functional changes by this patch.

Link: https://lore.kernel.org/r/20200103081714.9560-32-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>