virtio-serial-bus: use correct lengths in control_out() message

virtio-serial-bus: use correct lengths in control_out() message

Original code has one thing to process (cur_len), requests to
convert from iovec to buf another thing (len which is actually max_len),
and processes something else (copied).  Whole thing is very difficult
to understand, even if it does a right thing.  The iov_to_buf()
conversion in this case will always return cur_len, because it is
the length of the iovec it was asked to process, and the size we
asked to convert is the same or larger, and iov_to_buf() will stop
at reaching either iov or buf.

Make the code saner by doing the only sane thing: dropping `copied'
which is always the same as `cur_len' but just introduces questions.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>