V4L/DVB (4924): Fix some bugs on usbvision due to the merge into one module

Found the bug that prevented the driver from loading : a module
param conflict between usbvision-i2c and usbvision-core (debug
parameter).
- correct the module param "debug" conflics in usbvision-i2c.c and
usbvision-core.c
- add some debug printouts in usbvision-core.c VDIOC_QBUF/VIDIOC_DQBUF
- usbvision-core.c : add vb->field = V4L2_FIELD_NONE in VIDIOC_DQBUF

Signed-off-by: Thierry MERLE <thierry.merle@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>