[media] dvb-frontends: Add static

Add static to tda m_* variables in the header. They don't need to be global.
With some cleanup they could probably even be marked const.

Resolves-bug: https://bugzilla.kernel.org/show_bug.cgi?id=68191

Reported-by: Christian Schneider <christian@ch-sc.de>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>