media: dvb-frontends/mxl5xx: declare LIST_HEAD(mxllist) static

Fixes one sparse warning:
  mxl5xx.c:46:1: warning: symbol 'mxllist' was not declared. Should it be static?

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>