media: usb: siano: Use kmemdup to simplify kmalloc and memcpy logic

Using kmemdup() helper function rather than implementing it again
with kmalloc() + memcpy(), which improves the code readability.

Signed-off-by: Chen Jiahao <chenjiahao16@huawei.com>
Reviewed-by: Dongliang Mu <dzm91@hust.edu.cn>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>