media: gspca_zc3xx: Enable short exposure times for OV7648

The 50Hz and 60Hz power line frequency settings disable short (1/120s
and 1/100s) exposure times for banding filter (causing overexposed
image near lamps). No flicker setting enables them (when banding
filter is disabled and they're not used).

Seems that the logic is just the wrong way around.
(This bug came from the Windows driver.)

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>