media: mxl692: make a const array static, makes object smaller

Don't populate the const array fw_hdr on the stack but instead it
static. Makes the object code smaller by 5 bytes:

Before:
   text    data    bss     dec    hex filename
  31948   12072     64   44084   ac34 drivers/media/dvb-frontends/mxl692.o

After:
   text    data    bss     dec    hex filename
  31879   12136     64   44079   ac2f drivers/media/dvb-frontends/mxl692.o

(gcc version 10.3.0)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>