Store calculated setup_len in a local variable, verify it, and only
write it to the struct(USBDevice->setup_len)in case it passed the
sanity checks.
This prevent other code (do_token_{in,out}function specifically)
from working with invalid USBDevice->setup_len values and overruning
the USBDevice->setup_buf[] buffer.
Fixes: CVE-2020-14364
