ALSA: vx: vx_pcm: constify vx_pcm_playback_ops and vx_pcm_capture_ops.

File size before:
   text	   data	    bss	    dec	    hex	filename
   7126	    240	      0	   7366	   1cc6	sound/drivers/vx/vx_pcm.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   7382	      0	      0	   7382	   1cd6	sound/drivers/vx/vx_pcm.o

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