V4L/DVB: gspca_tv8532: rename brightness control to exposure

What we've called brightness so far actually is an exposure control,
rename it and fixup the maximum and default values.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>