ALSA : pcsp: pcsp_lib: constify snd_pcsp_playback_ops

File size before:
   text	   data	    bss	    dec	    hex	filename
   2027	    168	      1	   2196	    894	sound/drivers/pcsp/pcsp_lib.o

File size After:
   text	   data	    bss	    dec	    hex	filename
   2155	     40	      1	   2196	    894	sound/drivers/pcsp/pcsp_lib.o

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