V4L/DVB (7881): saa7134: fixed a compile warning in saa7134-core.c

patch 779169257208 made the variable mask unnecessary.
This patch just removes the declaration.

Signed-off-by: Hartmut Hackmann <hartmut.hackmann@t-online.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>