media: stv0910: declare global list_head stvlist static

Cleans up smatch warning:
symbol 'stvlist' was not declared. Should it be static?

Patch originally submitted by Colin Ian King <colin.king@canonical.com>,
remainder after the merge of all other stv0910 fixes.

Cc: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>