[media] several drivers: Fix a few gcc 4.6 warnings

au0828-dvb.c:99:6: warning: variable 'ptr' set but not used [-Wunused-but-set-variable]
au0828-video.c:1180:25: warning: variable 'maxheight' set but not used [-Wunused-but-set-variable]
au0828-video.c:1180:15: warning: variable 'maxwidth' set but not used [-Wunused-but-set-variable]
bttv-input.c:196:16: warning: variable 'current_jiffies' set but not used [-Wunused-but-set-variable]

Those variables are not used at all, so just remove them.

Cc: Steven Toth <stoth@hauppauge.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>