V4L/DVB: hdpvr-core: make module parameters local

The default_video_input and default_audio_input module parameters are
only used inside the hdpvr-core.c file so make them static.

This will remove the following sparse warnings (see "make C=1"):
 * warning: symbol 'default_video_input' was not declared. Should it be static?
 * warning: symbol 'default_audio_input' was not declared. Should it be static?

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