USB: visor: increase bulk-out buffer size

Increase the bulk-out buffer size to avoid any regression in throughput
after replacing the old writing scheme which used dynamic buffers (e.g.
up to 2k).

256b has been determined to be a good choice for several drivers
including ftdi_sio which used to have a more or less identical write
implementation.

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