media: usbvision: fix a potential NULL pointer dereference

In case usb_alloc_coherent fails, the fix returns -ENOMEM to
avoid a potential NULL pointer dereference.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>