usbvision: remove some unused vars

Gcc 6.1 warns about some unused vars. Remove them:

drivers/media/usb/usbvision/usbvision-core.c:94:18: warning: 'min_imgheight' defined but not used [-Wunused-const-variable=]
 static const int min_imgheight = MIN_FRAME_HEIGHT;
                  ^~~~~~~~~~~~~
drivers/media/usb/usbvision/usbvision-core.c:93:18: warning: 'min_imgwidth' defined but not used [-Wunused-const-variable=]
 static const int min_imgwidth = MIN_FRAME_WIDTH;
                  ^~~~~~~~~~~~
drivers/media/usb/usbvision/usbvision-core.c:92:18: warning: 'max_imgheight' defined but not used [-Wunused-const-variable=]
 static const int max_imgheight = MAX_FRAME_HEIGHT;
                  ^~~~~~~~~~~~~
drivers/media/usb/usbvision/usbvision-core.c:91:18: warning: 'max_imgwidth' defined but not used [-Wunused-const-variable=]
 static const int max_imgwidth = MAX_FRAME_WIDTH;
                  ^~~~~~~~~~~~

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>