ALSA: seq: oss: Constify snd_seq_oss_callback definitions

The snd_seq_oss_callback items are just copied to another struct
as-is, hence they can be declared as const.

There should be no functional changes by this patch.

Link: https://lore.kernel.org/r/20200103081714.9560-53-tiwai@suse.de


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