V4L/DVB (13688): au8522: modify the attributes of local filter coefficients

Make the local filter coefficients static and const. This will eliminate the
following sparse warnings (see "make C=1"):
 * au8522_decoder.c:71:31: warning: symbol 'filter_coef' was not declared. Should it be static?
 * au8522_decoder.c:113:31: warning: symbol 'lpfilter_coef' was not declared. Should it be static?

Signed-off-by: Márton Németh <nm127@freemail.hu>
Acked-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>