media: cxusb: pass buf as a const u8 * pointer and make buf static const

Don't populate the read-only u8 array buf on the stack at run time but
instead make it static const; makes object code smaller saving over 480
bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  33030	  65936	    192	  99158	  18356	drivers/media/usb/dvb-usb/cxusb.o

After:
   text	   data	    bss	    dec	    hex	filename
  32446	  66032	    192	  98670	  1816e	drivers/media/usb/dvb-usb/cxusb.o

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>