media: cx18: make array mapping static, makes object smaller

Don't populate the array mapping on the stack but instead make it
static. Makes the object code smaller by 79 bytes.

Before:
   text	   data	    bss	    dec	    hex	filename
  27572	   2096	      0	  29668	   73e4	drivers/media/pci/cx18/cx18-ioctl.o

After:
   text	   data	    bss	    dec	    hex	filename
  27429	   2160	      0	  29589	   7395	drivers/media/pci/cx18/cx18-ioctl.o

(gcc version 9.2.1, amd64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>