USB: visor: reimplement using generic framework

Kill custom read and write implementations (dynamically allocated write
urbs).

Note that I chose to remove the stat module parameter which was supposed
to keep count of the amount of data sent and received, but which has
been broken for three years (since b308e74d
"USB: visor driver adapted to new tty buffering" -- bytes_in was
incorrectly updated and was thus always reported as 0).

Compile-only tested.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>