V4L/DVB (8668): gspca: Conflict GSPCA / ET61X251 for the webcam 102c:6251.

Fix a double handling of 102c:6251 and no handling of 102c:6151
when both drivers GSPCA and ET61X251.

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