ALSA: es18xx: constify pnp_device_id and pnp_card_device_id

*_device_id are not supposed to change at runtime. All functions
working with *_device_id provided by <linux/pnp.h> work with
const *_device_id. So mark the non-const structs as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>