USB: usb_wwan/option: generalize option_send_setup for other drivers

Only the option driver implements the send_setup callback; it uses the
SET_CONTROL_LINE_STATE request in CDC ACM to generate DTR/RTS signals
on the port. This is not driver-specific though and is needed by other
drivers, so move the function to the usb_wwan driver (with formatting
tweaks), and replace the callback pointer with a flag that enables the
request.

Suggested-by: Bjørn Mork <bjorn@mork.no>
Suggested-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David Ward <david.ward@ll.mit.edu>
Signed-off-by: Johan Hovold <johan@kernel.org>