V4L/DVB (8605): gspca: Fix of gspca_zc3xx oops - 2.6.27-rc1

Bad mini/max check in setting control values (the gamma in
zc3xx could be set to null).

Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>