USB: ftdi_sio: switch to generic write implementation

Switch to the generic, multi-urb, write implementation.

Note that this will also make it fairly easy to use the generic
fifo-based write implementation: simply unset the multi_urb_write flag
and modify prepare_write_buffer (or unset if not using a legacy SIO
device). This may be desirable for instance on an embedded system where
optimal throughput at high baudrates may not be as important as other
factors (e.g. no allocations during runtime and less pressure on host
stack).

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