Allocate gspca_dev->usb_buf with kzalloc instead of kmalloc to
ensure it is property zeroed. This fixes various syzbot errors
about uninitialized data.

Syzbot links:

https://syzkaller.appspot.com/bug?extid=32310fc2aea76898d074
https://syzkaller.appspot.com/bug?extid=99706d6390be1ac542a2
https://syzkaller.appspot.com/bug?extid=64437af5c781a7f0e08e



Reported-and-tested-by:  <syzbot+32310fc2aea76898d074@syzkaller.appspotmail.com>
Reported-and-tested-by:  <syzbot+99706d6390be1ac542a2@syzkaller.appspotmail.com>
Reported-and-tested-by:  <syzbot+64437af5c781a7f0e08e@syzkaller.appspotmail.com>

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>