usb: uvc: respect setup->wLength in responses

Due to the alignment and granularity requirements of memory allocation,
setup->wLength is shorter than the allocated buffer size.
This lead to responses larger than what the host requested, which it
rejected. Fix it by using the minimum between the allocated size, the
struct size, and the wLength requested.

Signed-off-by: Josuah Demangeon <me@josuah.net>