tests: subsys: usb: bos: Remove dummy_descriptor struct

When building with clang, it warns:

tests/subsys/usb/bos/src/test_bos.c:24:22: error: variable
'dummy_descriptor' is not needed and will not be emitted
[-Werror,-Wunneeded-internal-declaration]
static const uint8_t dummy_descriptor[] = {
                     ^

Only the size of dummy_descriptor is used, not the contents. Replace the
struct with a constant for the size.

Signed-off-by: Tom Hughes <tomhughes@chromium.org>