[media] gspca - ov519: Fix a regression for ovfx2 webcams

By git commit c42cedbb658b, the bulk transfer size was changed to a lower
value for resolutions != 1600x1200, but the image extraction routine still
worked with the previous value, giving bad truncated images.

Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>